Journal of Molecular Liquids, 34 ( 1987) 269-283 ElsevierSciencePublishersB.V.,Amsterdam-PrintedinThe THE DYNAMICS OF LIQUID WATER : ELECTRIC 269 Netherlands FIELD EFFECTS M. W. EVANS* University College of Swansea, * Current Address: (Received Singleton IBM, Dept. 48B/428, 22 September Park, Swansea, Neighborhood SA2 8PF Rd., Kingston, NY 12401,U.S.A. 1986) ABSTRACT The technique investigate of field-effect the decoupling related non-linear adapted for this purpose transient effect; effects acceleration computer the fall transient in liquid water at electric Langevin function. This is interpreted acceleration; and Using a pair potential and persistent; field strengths to mean and that the system has been used to ST2, it is found that fall is pronounced decoupling are very strong, at 300 K. from the well known in water moderate water simulation sufficient and accompanies to saturate that non-linear is statistically a the effects in non-Markovian and non-Gaussian. INTRODUCTION The technique of field-effect computer identify and characterise matter. These are the field decoupling Evans Cl1 article and confirmed [21 using the modifications fundamental try to explain framework, recently of motion, Langevin the decoupling effect means effect analytically 0 1987ElsevierSciencePub1ishersB.V. link between and the equation falls outside at all. that I51 as Reduced equation, If the diffusion the effect might not be predicted 0167-7322/87/$03.50 and shows in detail a rigorous such as the Liouville equations. C31 and theory in order to begin summarised seems to be able to provide acceleration A review methods. The field decoupling class of equations, equations much simpler-to-solve diffusion state of by and co-workers and numerical to conventional of the new effects. (R.M.T.), theoretically by Oxtoby on the work in this area [41 necessary only one particular Model Theory, algorithms Cl] has been used to of the condensed effect and fall transient numerically independent is now available to take account simulation properties The first of these was predicted or deceleration. Grigolini two general used to the R.M.T. For example, the the 270 diffusion elegant equation solution reproduce used by Praestgaard to the problem the decoupling with the inherently acceleration removal or deceleration from a molecular difficult to describe. acceleration potential nature potential in nature. at all it is necessary ensembles involved of the fall transient orientational following between C41 in comparison Furthermore the acceleration behaviour or using strong fields in a numerical theory of molecular at all, C71 Furthermore, strong applied Debye's to inter - molecular diffusion other alteration possible electric equations dissipation simulation. a fall transient C81,either a decoupling For theorem holds rigorously. effects and adapted effect for arbitrary effect [2,51 equations of but if no diffusion, it becomes but there is still no and equilibrium dependence no coefficient function, of rotational The fall transient but linear diffusion acceleration D.C. or A.C. If the friction by a memory the field decoupling in this case retain an identical for non-Markovian field The effect is not a is that the theory contains potentials. is replaced acceleration. equilibrium for inertial fields, is made to the equation to describe fall transient a.c.f. - will not provide The reason 171 or deceleration theory of Debye - the great "classical" theory will not provide field strength. reference Debye's diffusion ensemble. even when the theory is corrected for arbitrarily external of the molecular diffusion with tells us a lot about the normal, free, equilibrium the rotational equations or rotational with the appropriate function and interacting the fall transient this must be done for banal consequence example, on the recognition Langevin in the translational In other words auto-correlation the instantaneous diffusing to supplement The field is much more In order to describe in that equation. at equilibrium. and but does not of the liquid state of matter. of a strong applied terms that are -non-linear coordinates by these authors, It relies for its description is not linear is a plausible It is known now that this has to do of fall transients ensemble of the fact that the effective molecules considered effect at all. non-Markovian 161 and van Kampen upon time. orientational In other words the first fluctuation - The same is true for translational diffusion. The level of sophistication primary of fall transient acceleration side of these diffusion effective is reached equations inter-molecular which irrespective translational coordinate, or not with a memory 141 for an analytical by a representation is non-linear of whether function. description when the terms on the left hand are supplemented potential been replaced v = - v. cos8(t) needed of the in the rotational the friction coefficient One of the simplest or has of these is: (1) 271 which could for example between two molecular This automatically represent dipoles the analytical 0 $'(t-r)8(r)dr + Vosin8(t) where $ is the memory Mori, C91,and W(t) is a Wiener rotational function diffusion an elementary acceleration analysis. coordinate of the However, the original An equation the introduction equation of a model inside a diffusing the diffusion truncation of the angular velocity the full equivalence linear itinerant oscillator when the original so called "non-linear representation subjected and co-workers to describe numerically simulation behaviour using algorithms. model" pair potential have shown C41 function. Recent work Cl47 and also that the cage and encaged molecule In this eventuality becomes a relatively diffusion the simple of molecules that this model can be used and decelerations molecular dynamics link between pair - potential. of this type are necessary, predictions fraction of type (1). three fully independent work of Grigolini an and more transparent accelerations and the effective Cl11 bound harmonically corresponded This has forged an analytical and analytical the theoretical between of the rotational observations oscillator. of the Mori continued correlation oscillator level of the in two dimensions of type (1). fall transient of fall transients that real experimental numerical itinerant in two dimensions Grigolini of a molecule of the two approaches, binding potential to an inter-molecular successfully observed harmonic as solving can be met, itinerant takes on a more plausible by a non-linear Cl01 by Coffey and Calderwood that this model has demonstrated is replaced almost as difficult cage, treated as a rigid entity, to a particular the need analysis to go to the next higher motion into the implies and moment This difficulty first introduced It was shown by Evans Cl21 Cl31 becomes "ab initio". it is possible and aimed at describing expansion function procedures, C51 and to deal with the non-linear framework This is a version mathematically and fall transient field is introduced of the memory (2), so that the problem but alternatively, annulus. electric truncation by the random torque of the of type (2) is capable of providing of both the decoupling fractions, Liouville type first introduced representing effects when the external to solve eqn. meaning t. to two dimensions. = W(t) of the simple process equation. description to use continued R.M.T. them at the instant consideration 0 is also the rotational t I energy equation: B(t) + Cl01 of the potential on the angle 8 between restricts In this case the coordinate Langevin the dependence in real molecular and co-workers liquids. [4,5,131 computer the It follows to test the Furthermore, and of Coffey and 212 co-workers [11,141 apparently disparate with as V. of eqn (1). acceleration paper, this model has shown how it may be used to inter-relate phenomena Among effects. in terms of a few well defined these are the decoupling This implies Section methods and of the parameters subjected to saturate done in practice radiation electric field strengths measurements remains Lennard function. that characterises In section the principal The original molecular moment noting effect. tetrahedral of inertia that the analytical to show up deviations in particular from function computer acceleration and that In this respect we . and Rahman disposition Cl81 to of partial charges 1191 in terms of the fall transients to coincide with the I axis of frame. velocity The decoupling effect is in axes 1,2 and 3 of the moment it is shown that the complete is statistically description equation non-Markovian of the computer that is non-linear, These three characteristics in section the a.c.f. 's such as those of the angular velocity, using the model of the non-linear Finally of Stillinger in this section have to rely on a diffusion also non-Gaussian. even with Cl61 accurate auto correlation cl71 by other methods. J+,,which happens a.c.f. 's investigated This implies with electro- However, to liquids such as water are presented and rotational Finally frame This could not be of .such investigations, work ST2 has been retained. using equilibrium frame. equilibrium absence and Jones terms, and this gets rid of their artificial 11 the results angular momentum, inertia orientational the simulation of the water dipole moment, illustrated might be expected exhaustively in terms of algorithms in Kerr effect experiments, the simple ST2 potential involve atom-atom function. the only way of looking at fall transient to extend have been investigated have adapted simulation laser technology. of the equivalent In the continued it is important switching available in this of 108 water molecules field, but might be plausible on the fall transient at equilibrium. simulation the Langevin using contemporary the time dependence dynamics field in the z axis of the laboratory Cl,151 with electric magnetic the computer the molecular to a static electric strong enough to be described self consistently it contains. 1 of this paper describes used to investigate such and fall transient that the results for water at 300 K, could also be described the same model, parameters itinerant of set of in nature. simulation non Markovian, will and can now be inter-related [Sl oscillator. three some suggestions the need for experimental for further work are given, data on the fall transient 273 SECTION 1: COMPUTER A complete SIMHLATION description those of Stillinger available of the algorithm employed and comparison of results with Cl81 and of Clementi et al., C201 and Rahman in an accompanying the methods ALGORITHM paper. in treating Here we restrict Cl91 is the description the sample with very intense external to electric fields. The key to such methods Cl1 is that the kinetic is always kept constant, both during place in the picoseconds after the field is applied, transient process is achieved easy to control energy) during equilibrium no different in a standard the fall transient of the external strong enough release external thermodynamic transient Table dynamics does not affect resealing C211 and the equivalent ensembles kinetic terms part of the Hamiltonian, is reduced considerably by the in the ensemble. to equilibrium infinitesimally This theorem actually equilibrium With after its dissipation-theorem, removed implies from that the fall a.c.f. must be identical in their 1 Mean Energies, Kinetic Field Off and Field On. Potential (kJ / mole) (kJ / mole) This It is at field free In energetic at all by the first fluctuation for statistical equilibrium. field. routines. here, and the procedure the kinetic the regression takes the fall and translational simulation. torque -8xg on each molecule fields $ which of the electric of interest in strong contrast, cannot be described which was derived process process, and also during from the control of temperature molecular process energy, removal temperature (i.e. the rotational the fall transient but the potential release with standard the temperature is, in actuality, the rise transient after the instantaneous numerically energy of the ensemble uE(kJ / mole) Translation Rotational 3.78 3.88 -35.77 3.85 3.76 -35.27 0.50 3.85 3.81 -25.12 10.65 3.85 3.74 -17.34 18.43 3.90 3.73 -15.58 20.19 0.00 214 time dependence. investigations perturbation Note that this is assumed of molecular dynamics of the ensemble The application, with external therefore, implicitly in all the experimental based on spectroscopy 122,231 i.e. the radiation. of a z axis electric field &, produces the torque -txE on each H 0 molecule. If the unit vector E, is defined in %Z 2 the dipole moment axis t the equilibrium average <e12(t)> becomes non-zero a result of this torque. over the 108 molecules Here the average at each instant dependence of this average saturates, Cl1 can be removed t after field application. instantaneously to generate was continuously table 1, together with the rotational the external the fall transient. monitored The This eventually level, after which a plateau as is just a simple average on time is the rise transient. i.e. reaches energy in the simulation <e12(t)> and is summarised and translational kinetic field The potential in energies. RESULTS AND DISCUSSION With the use of computer Langevin function saturation for water simulation This function (fig. (1)). level of each rise transient this figure is the theoretical Langevin it is possible plotted function against to saturate was constructed uE/kT. the from the The curve in and the points are obtained L Fig. 1. The Langevin a) of rise transients b) Rise transients (1) - (5) from the computer <eZ> Increasing corresponding function simulation Saturation see fig. (lb). plotted vs. time in picoseconds. field strength; respectively L (nE/kT). (6) - (7) fall transients to (3) and (2). levels ‘IO from the simulation stronger Fig. dipole (fig. (lb)). the field the shorter The rise transients (2) plots three fall transients a.c.f. the a.c.f. and are therefore large and is also persistent. characteristic available I I Fig. 2. water Field-off Table water 1 potential (potential the strongest constant Although energy C Qtr and translational = - 35.77 kJ/mole) field the atom-atom applied for the field free in the presence The sample of is clearly of liquid water. of the H20 molecule <l$(t)~T(0)>/(<W2(0)> t <v2(o)> 11 This field and the fall - transient a property functions field. between is small. ways of seeing this is through time cross-correlation in of fig. (3). structure of the strongeat energy functions p.d.f. 's are little affected properties kinetic and equilibrium (-15.58 kJ/mole) of these is: = function for all E, but the mean pair distribution in equilibrium of fig. (2) is therefore field, the dynamical simplest auto-correlation in this simulation by the atom-atom applied One of the clearest field induced dipole is more than halved by the strongest still a liquid in the presence acceleration with , equilibrium These show that the difference case can be detected Cl61 0.24 1 shows that the rotational energy is very that this is field strength. is approximately point is emphasised to it for effect . . . . . . . Fall transients <es> , (see text). <e(t).y(o)>/<p'>. (l)-(4): Decreasing with respect and that these accelerations 0.12 equilibrium to zero than acceleration to assume field Kerr effect apparatus. 0 - the at field free equilibrium. considerably The fall transient It is reasonable of real water, electric the appropriate decay much more rapidly accelerated shown in the figure. against of course, computed, <#t).~(o)>/+ It can be seen that the fall transients all pE/kT are field dependent the rise time. even by the strongest are changed significantly. the appearance C24-271 (c.c.f.'s). of electric The 276 Atom atom pair distribution Fig. 3. Field-off . . . . where Field-on equilibrium equilibrium the subscript T arguments. (potential denotes have shown analytically symmetry (potential that $& However vector functions. energy = -35.77 kJ/mole) energy = -15.58 kJ/mole). transposition, vanishes RycLaert for all t when B = 2 fig. (4) shows clearly et al. [281 using general that the (x,y) and (y,x) elements of C appear directly in the laboratory frame (x,y,z) for E > 2 str liquid water. This result confirms others in the literature C241 obtained using two independent speaking therefore, condensed molecular numerical computer cross correlation matter whenever simulation functions the sample algorithms. is subjected Fig. 4. (x,y) and (y,z) elements of the cross-correlation <wy(t)vxW <oxWvy(o)> (X,Y) z I ; (y,x): , I .,~,o)>f<";,o)>~ <p(o)>~<"~(o,>~ at field on equilibrium, Strictly such as these appear energy as for fig. (3). in to a symmetry function C Qtr in (x,y) and (y,x) elements Fig. 5. <(e(t) (X,Y) 2 at field-on breaking liquids ; in dielectric takes no account in frame (x,y,z). C25-271, that may exist in molecular Another moments example of is shown of the second order c.c.f. <lJJ2(o)><v2(o+ Coriolis In fig. (x,y,z). of fig. acceleration (5) the two elements and its own angular velocity again seem to be mirror to pick up from the noise images, EFFECTS in this section. frame a.c.f.'s, The decoupling and some of these are effect in than the mirror-image (4). These show up in laboratory illustrated and Kerr effect x $t)&T(O)> the molecular THE DECOUPLING c.c.f.'s (1,2,3) of the principal or for $ > 2, directly = diffusion Recent work has shown furthermore (5) in the shape of two elements but are much more difficult elements etc. theory of rotational in frame <t(t) frame ; field, as, for example, The simple of these effects. c,(t) between 1 is only one out of very many possible inertia, in fig, x ~w)x~yb)’ <w*(o)xv2(o)> external that C %tr molecular $2 equilibrium. spectroscopy. whatsoever of the cross-correlation can be identified by 278 looking at the envelope of the oscillations induced by the external It is present when the time dependence of this envelope than that of the equivalent a.c.f. C61 fail to produce analytical models inherently Markovian independent virtually in statistical analytical no attempt investigation and numerical frequencies, in a liquid and monitoring fast detector well-defined Fig. 6. confirmations, however, the former are The effect has received many [4,51 but there has been Its experimental by the use of induced birefringence using a giga watt laser to induce birefringence with a submillimeter systems. theoretically is out of balance this effect because nature. The present and numerically, laser or an interferometer situation, where and un-investigated of the decoupling functions. 1. Angular velocity a.c.f. at field off equilibrium. 2. Angular velocity a.c.f. at field on equilibrium. . . . . . . Envelope of the field oscillations Rotational kinetic energy a.c.f. at field off equilibrium. 4. Rotational kinetic energy a.c.f. at field on equilibrium. of the field oscillations the extension field free time dependence a.c.f. <x(0).8(t)> <W2> <t(t) in 4). of the oscillation envelope of the a.c.f. is visible with respect .$gt)@(o) .~(O)>/d> to the in the angular velocity (fig. (6)) and in the second order rotational energy a.c.f. auto- in 2). 3. For water is experimentally, effect with the angular velocity correlation Envelope with the phenomenon and unsatisfactory. Illustration ---_- field. to be slower It is known now that some to look for it experimentally. could be achieved, at far infra-red suitably field-free begins kinetic 279 1.0 1.0 _$.5 d 0.5 d 0 0 OO -0.5 0 0.2 tcpS1 0.1 1.0 0.05 0 ICI ,D 0.5 ‘t. $0 -0.5 + 0 0.05 0.10 tcps1 Fig. 7. a) Field decoupling effect in <,$1(t) . &T(O)>/ <&f > ; b) Field decoupling effect in c,&(t) . g2(0)>/ <eg > ; cl Field decoupling effect in c&(t) . &(o)>/< 62 > . Fig. 8. Field decoupling effect 3 in the angular moment a.c.f. 0.10 t(ps1 280 It is confirmed in the three rotational <&W . &(o)>/<q> ; 4,(t) . &(o)>/<e,2 > ; <g,(t) . $3(o)>/42> ; (fig. (7)); in the angular momentum angular all aspects of water, E simultaneously of the molecular needing investigation to express [4,14l acceleration, the centre of mass linear velocity, velocity, motion experimental a.c.f.'s (fig. (8)) and in the first and a.c.f. second order a.c.f. 's of the Coriolis considers velocity C25-271 which x, and the resultant (fig. (9)). Therefore and is clearly Our future investigation. and to provide results interpretation occurs in property analytical itinerant in terms of the parameters a self consistent molecular decoupling a fundamental of these results will use the non-linear these numerical automatically oscillator of that model of the results of this paper. Fig. 9. Field decoupling Coriolis acceleration NON-MARKOVIAN in the first and second order a.c.f.'s AND NON-GAUSSIAN PROPERTIES OF LIQUID WATER This section deals finally with these properties, and defined in the literature. [4,12,14,22,231 properties of liquid water have been known Stillinger and Rahman. velocity a.c.f. 118,19,201 of the (see text). Cl81 in frames which dynamical since the first simulations It is well known, for example, (x,y,z) and (1,2,3) has a negative and is not a simple exponential. are characterised The non-Markovian by that the linear overshoot The simple Langevin equation would 281 produce Doob's an exponential theorem. the whole decay, [22,231 range of correlation The non-Gaussian Berne and Harpe This property [291 nature can be detected and comparing a.c.f. C41 confirms that the dynamics in the well-defined in describing the evolution thermodynamic equilibrium Clearly capable frame rigorously Gaussian for this work and the in the liquid state at 300 K are sense that Gaussian of describing builds statistics do not succeed self-consistently. becomes this assumption data reported the complete At Gaussian into the here must be interpreted range of new phenomena capable (1,2,3). in the absence implies (1,2,3) [4,141 the existence For example, of which A of an electric This implies (1,2,3) and these are illustrated elements) must vanish out of the three vectors therefore theoretically the external in the product on the grounds electric field. that the diagonal and all elements are equal. simulation of molecular in survives Cl91 of this tensor must vanish for all the molecules shows that the computer which elsewhere. This is indeed what is found in the simulation. the tripleproduct functions is reducible is the only property that all other elements field of cross correlation the triple product 1 of anisotropy. (the off diagonal self itinerant of this. liquid sample in the absence exist in frame The group theory exist from the first order Future work aims to show that the non-linear after averaging theory analytically assuming simulation (1,2,3) as nine elements, (x,y,r). such (i.e. t + m) their behaviour is qualitatively in the moving in frame second order a.c.f.'s energy: of water theory can be used to predict elements by liquid. of these two a.c.f.'s computer For an isotropic frame in nature. for carbon monoxide out this exercise the range of numerical consistently. group non-Markovian C301 using a model oscillator simulation <~(t).F(O)>I& not Gaussian, algorithm. through show that liquids was first detected that calculated We have carried once more, because process for liquid water is similarly by investigating kinetic the result with linear velocity statistics. of the Markov . I>/&, . x(t)x(o) <x(t) functions of molecular using computer as that of the translational result characteristic These and other results in frame At the time origin in the ensemble because The application of group provides symmetry the elements in the absence of two which 282 ACKNOWLEDGEMENTS The University Fellowship of Wales is thanked for the award of the Pilcher and IBM for the award of a Visiting Senior Professorship. REFERENCES 1 M.W. Evans, "Molecular "Memory Function vol 62 of Advances (Wiley/Interscience, Physics, New York, P. Grigolini, 3 B. Bagchi and D.W. Oxtoby, 4 M. Ferrario, Mol. Phys., ed. M.W. Evans, P. Grigolini and and S.A. Rice, J. Phys. Chem., 86 (1982) 2197; ibid., 77, (1982) 1391 P. Grigolini, A. Tani, R. Vallauri and B. Zambon, ref. (l), VI. P. Grigolini E. Praestgaard 7 M.W. Evans, 8 W.T. Coffey and B.V. Praanjape, 9 H. Mori, Prog. Theor. Phys., in ref. (l), Chapter 1. and N.G. van Kampen, Phys. Rev., A, 30(4) Mol. Phys., Proc. Roy. Irish Acad., G. Gross0 and G. Pastori-Parravincini, J.H. Calderwood 12 M.W. Evans, Chem. Phys. Lett., 3 and 4. 39 (1976) 601 and F. Marchesoni, to Adiabatic "Development Motion", in "Dynamical Advances in Chemical and S.A. Rice, ref (l), chapter and W.T. Coffey, Proc. Roy. Sot., A 356 (1977) 269 of Rules Leading W.T. Coffey, 78A (1978) 17 33 (1965) 423; 34 (1965) 399 11 P. Grigolini 43 (1981) 33 (1984) 2062 10 ref. (1) chapter 2 : "Basic Description Elimination". and Application Processes Physics, of the Theory of Brownian in Condensed ed. M.W. Evans, (Wiley/Interscience, 15 M.W. Evans, J. Chem. Phys., 16 M.S. Beevers 17 D. Bertolini, 18 F.H. Stillinger 19 G.J. Evans, M.W. Evans, P. Minguzzi, in ref. Matter", 31 (1976) 1717 6 14 in Condensed 1985), Chapter v. 5 13 Problems series ed. I. Prigogine 2 Chapter to Stochastic in Chemical G. Pastori-Parravicini, J. Chem. Phys., : Intense External Fields", in Dynamics Approaches New York, Matter", 1985), p. 63 ff. 76 (1982) 5473 and D.A. Elliot, Mol. 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