Iwao Ohmine

Molecular dynamics simulation of the ice nucleation and growth process leading to water freezing

Upon cooling, water freezes to ice. This familiar phase transition occurs widely in nature, yet unlike the freezing of simple liquids, it has never been successfully simulated on a computer. The difficulty lies with the fact that hydrogen bonding between individual water molecules yields a disordered three-dimensional hydrogen-bond network whose rugged and complex global potential energy surface permits a large number of possible network configurations. As a result, it is very challenging to reproduce the freezing of ‘real’ water into a solid with a unique crystalline structure. For systems with a limited number of possible disordered hydrogen-bond network structures, such as confined water, it is relatively easy to locate a pathway from a liquid state to a crystalline structure. For pure and spatially unconfined water, however, molecular dynamics simulations of freezing are severely hampered by the large number of possible network configurations that exist. Here we present a molecular dynamics trajectory that captures the molecular processes involved in the freezing of pure water. We find that ice nucleation occurs once a sufficient number of relatively long-lived hydrogen bonds develop spontaneously at the same location to form a fairly compact initial nucleus. The initial nucleus then slowly changes shape and size until it reaches a stage that allows rapid expansion, resulting in crystallization of the entire system.

Instantaneous normal mode analysis of liquid water

We present an instantaneous‐normal‐mode analysis of liquid water at room temperature based on a computer simulated set of liquid configurations and we compare the results to analogous inherent‐structure calculations. The separate translational and rotational contributions to each instantaneous normal mode are first obtained by computing the appropriate projectors from the eigenvectors. The extent of localization of the different kinds of modes is then quantified with the aid of the inverse participation ratio—roughly the reciprocal of the number of degrees of freedom involved in each mode. The instantaneous normal modes also carry along with them an implicit picture of how the topography of the potential surface changes as one moves from point to point in the very‐high dimensional configuration space of a liquid. To help us understand this topography, we use the instantaneous normal modes to compute the predicted heights and locations of the nearest extrema of the potential. The net result is that in liquid water, at least, it is the low frequency modes that seem to reflect the largest‐scale structural transitions. The detailed dynamics of such transitions are probably outside of the instantaneous‐normal‐mode formalism, but we do find that short‐time dynamical quantities, such as the angular velocity autocorrelation functions, are described extraordinarily well by the instantaneous modes.

Large local energy fluctuations in water. II. Cooperative motions and fluctuations

Large local energy fluctuations in liquid water and their physical origin are investigated by using classical molecular dynamics (MD) calculation and quenching techniques. Performing a trajectory calculation of 100 ps, it is found that large rotational motions of individual water molecules, which are always associated with potential energy destabilization of 10–20 kcal/mol, occur once in about 10 ps. The stabilization and destabilization of the individual water molecules are induced by cooperative motions. In order to analyze these cooperative motions in the liquid water, the water structures are quenched to their local minima (called the inherent structures). Comparing the inherent structures successively visited by the system, it is found that collective motions of about 10–40 molecules localized in space occur in unstable regions. The potential energy fluctuation of an individual molecule can reach up to 15 kcal/mol even in the inherent structures. The strong potential energy correlation among neighboring molecules indicates these cooperative motions cause the ‘‘flip–flop’’‐type energy exchanges; as a molecule is stabilized, another is to be unstabilized and vice versa. A flip‐flop motion does not involve a (large) energy barrier but causes large energy fluctuations of the individual molecules. A large portion of potential energy fluctuations of the individual water molecules is accounted for as the superposition of fluctuations in the inherent structures and those in the normal modes build upon these structures.

Structure, dynamics, and thermodynamics of model (H2O)8 and (H2O)20 clusters

We present molecular dynamics simulations of (H2O)8 and (H2O)20, paying particular attention to the possibility of solid‐like/liquid‐like coexistence. Four differently parametrized rigid molecule potentials are examined for (H2O)8; only the most promising is applied to (H2O)20. In every case, we find evidence for time‐scale coexistence in the statistics of the short‐time averaged temperature. In several cases, we also observe loops in the microcanonical caloric curve [T(E)], indicating the formal existence of two stable states over a finite range of energy. Further evidence is provided by systematic quenching, both by comparison with the dynamics and in terms of model density of states calculations of the microcanonical T(E), energy distribution function f(E), Helmholtz free energy A(T), and heat capacity Cv(T). We discuss two possible approaches to these thermodynamic functions from the distribution of local energy minima and compare the results with those for atomic clusters bound by the Lennard‐Jones p...

OFF-RESONANT FIFTH-ORDER NONLINEAR RESPONSE OF WATER AND CS2 : ANALYSIS BASED ON NORMAL MODES

Off-resonant fifth-order nonlinear response functions of liquid water and liquid CS2 are analyzed based on two normal-mode schemes, quenched and instantaneous normal modes. It was found that the fifth-order response function is very sensitive to the mode mixing in polarization, arising from the quadratic term of polarization with respect to the different modes. The echo signal is drastically reduced by this off-diagonal mode mixing in polarization even without any rapid frequency modulation mechanism. The near absence of echo signal thus obtained for liquids is consistent with the recent experimental results for liquid CS2. The present calculation yields the different fifth-order signals for different polarization geometries, as experimentally shown by Tokmakoff and Fleming [J. Chem. Phys. 106, 2569 (1997)]. The mode mixing dynamics is investigated in terms of the bispectra of total potential energy and polarizability.

Large local energy fluctuations in water

A detailed analysis is made for the dynamical behavior of an individual water molecule in liquid water by using a classical molecular dynamics (MD) calculation. It is found that there exist very large potential energy fluctuations in water; a single water molecule can exhibit a fluctuation of the order of 10∼20 kcal/mol. These potential energy fluctuations can be classified into two categories; the fast component (10−14–10−13 s) associated with librational motions of water molecules and the slow component (10−12–10−11 s) associated with water binding structure changes. Both amplitudes can be reached up to 10 kcal/mol. Due to strong Coulomb (dipole–dipole) interaction, small mutual geometrical changes, caused by the libration motions, induce large fast potential energy fluctuations. Due to large cohesion energy of the hydrogen bond and the nature of the water binding structure, there exist many water pair interactions which are unattactive or even repulsive; the water molecule potential (binding) energy di...

Mechanisms of nonadiabatic transitions in photoisomerization processes of conjugated molecules: Role of hydrogen migrations

Mechanism of nonadiabatic transition in the C=C torsional photoisomerization process of ethylene and polyenes is investigated by using the ab initio configuration interaction calculation method. We have calculated the low‐lying singlet state potential energy surfaces and their nonadiabatic couplings. A multidimensional search for the molecular configurations yielding strong nonadiabatic couplings is performed to find the origin of very fast photoisomerization kinetics, which are experimentally observed to be typically in the order of a few or a few tens of picoseconds. It is found that the ‘‘pseudo’’ migration motion of a hydrogen adjacent to the twisted C=C bond causes a potential surface crossing of the low‐lying excited and ground states and thus induces a sufficiently large nonadiabatic coupling to explain this experimental evidence. The hydrogen migration motion is facilitated by the so‐called zwitterionic character of the low‐lying excited states near the 90° C=C twisted conformation, proceeds almos...

Rearrangements of model (H2O)8 and (H2O)20 clusters

We have calculated rearrangement mechanisms for (H2O)8 and (H2O)20 clusters by eigenvector following. For (H2O)8, two different parametrizations of a four‐site, rigid water effective pair potential were considered and found to give very similar results. Hence, only one of the potentials is applied to (H2O)20. 6N−6 internal coordinates are required to describe a (H2O)N cluster in these calculations, of which 3N−6 were chosen as center‐of‐mass distances, angles, and dihedral angles, the other 3N being Euler angles. A wide variety of different rearrangements for both (H2O)8 and (H2O)20 are illustrated, with barrier heights ranging over three orders of magnitude. The mechanisms range from almost imperceptible changes of geometry to folding processes that result in dramatic structural transformations.

Potential energy surfaces for water dynamics. II. Vibrational mode excitations, mixing, and relaxations

Dynamical behavior of liquid water is investigated by analyzing the potential energy surface involved. Multidimensional properties of the potential energy surface are explored in terms of vibrational mode excitations at its local energy minima, called inherent structures. The vibrational mode dynamics, especially mechanism of mode relaxation and structure transitions, is analyzed. It shows very strong excitation energy dependence and mode dependence. There are three kinds of vibrational coupling among modes. For excitations of energy near the room temperature, most modes (more than 90% of total modes) individually interact with only one or two other modes, and yield near recurrence of the mode energy in a few tens picoseconds (very slow relaxation). Spatially localized modes in the intermediate frequency range couple with many delocalized modes, yielding fast relaxation. The coupling is governed by atomic displacement overlaps and frequency matching. Each mode couples with nearby frequency or double frequ...

Nanosecond laser photolysis of the benzene monomer and eximer

A dilute solution of benzene was photolyzed with a KrF excimer laser. The Sn←S1 and Tn←T1 absorption spectra are measured with time‐resolved spectroscopy. The Sn←S1 absorptions in the energy range between 850 and 220 nm were analyzed. The transient absorption band at 500 nm, which was observed previously by several workers disappeared completely upon dilution of the sample, and thus was assigned to be the excimer band. We assigned the other states as follows: 620 nm (1E1u, f∼0.003), 540–325 nm (11E2g, f∼0.04), and 270 nm (21E2g, f∼0.12). The observed energies from the ground state to the 11E2g and 21E2g states 7.8 and 9.2 eV, respectively. The Tn←T1 spectrum showed a single peak at 235 nm (f∼0.35) and a shoulder around 310 nm (f∼0.12).