Nonrigid Molecular Systems Research Articles

Tunneling Splittings in Water Clusters from Path Integral Molecular Dynamics.

We present calculations of tunneling splittings in selected small water clusters, based on a recently developed path integral molecular dynamics (PIMD) method. The ground-rotational-state tunneling motions associated with the largest splittings in the water dimer, trimer, and hexamer are considered, and we show that the PIMD predictions are in very good agreement with benchmark quantum and experimental results. As the tunneling spectra are highly sensitive to both the details of the quantum dynamics and the potential energy surface, our calculations are a validation of the MB-Pol surface as well as the accuracy of PIMD. The favorable scaling of PIMD with system size paves the way for calculations of tunneling splittings in large, nonrigid molecular systems with motions that cannot be treated accurately by other methods, such as the semiclassical instanton.

Operators of physical quantities of nonrigid molecular systems with continuous axial symmetry groups

The construction of the effective operators of physical quantities is considered for nonrigid molecular systems whose geometrical symmetry of internal dynamics is determined by continuous axial groups. The construction is based on the methods of a symmetry group chain.

Classification of the states of nonrigid molecular systems with continuous axial symmetry groups

Construction of the stationary state classification is considered for nonrigid molecular systems whose geometrical symmetry of internal dynamics is determined by continuous axial groups. The construction is based on the methods of symmetry group chains.

The differentiable manifold model of quantum-chemical reaction networks

The methods of general and differential topology (topology ≈ “rubber geometry”) are particularly suitable for the quantum-mechanical description of nonrigid molecular systems. The concept of nuclear configuration space 3NR is replaced by a metric space M and subsequently by a topological space (M, TC) where points of nuclear geometries, as fundamental entities, are replaced by open sets. These open sets provide a quantum-mechanical description of molecular structures, reaction mechanisms, and reaction networks. Exploiting the special properties of the metric of M, the topological space (M, TC) can be provided with a differentiable manifold structure. This differentiable manifold is proposed as a common basis for both local and global analysis of reacting molecular systems.

Application of the instanton method for analyzing tunneling splitting spectra of nonrigid molecular systems: II. Excited states

For nonrigid molecules with two equivalent minima on their potential energy surface, expressions are obtained in terms of the instanton method for the calculation of the magnitude of the tunneling splitting of vibrational levels with the number n≥0 both in the harmonic approximation and taking into account the anharmonic corrections ∼ ℏ2. Using comparative calculations of the lowest spectral region of the tunneling splitting of the NH3 molecule as an example, it is shown that, even in the harmonic approximation, the calculation accuracy of the instanton method is as good as that of the standard Wentzel-Kramers-Brillouin method. When the anharmonic corrections to the energy levels are taken into account, the accuracy of description of the tunneling splittings of the vibration-inversion spectrum of molecules of the NH3 type in terms of the instanton method increases almost to the level of quantitative agreement with experimental data.

Application of the instanton method for analyzing tunneling splitting spectra of nonrigid molecular systems

General formalism and basic ideas of the instanton method are described. The potentialities of the method are demonstrated by calculating the tunneling splitting of the ground vibrational level for nonrigid molecules with two and three equivalent minima on their potential-energy surface. Some advantages of the application of the instanton method as compared to the well-known semiclassical approximation for describing the spectra of tunneling splitting of a nonrigid molecular system are proved.

Structure and Dynamics of Non-Rigid Molecular Systems

Structure and Dynamics of Non-Rigid Molecular Systems

Photoinduced processes in structurally nonrigid molecular systems based on the pyridinium cation and salicylidenaniline molecule

Photoinduced processes in structurally nonrigid molecular systems based on the pyridinium cation and salicylidenaniline molecule

Application of Periodic-Shell Boundary Condition to Non-Rigid Molecular Systems. Molecular Dynamics Simulations on a Flow around a Circular Cylinder.

The present paper discusses the availability of the periodic shell boundary condition, which the author developed in the previous paper as an outer boundary condition for molecular dynamics simulations, to nonrigid molecular systems. For this purpose, we have conducted molecular dynamics simulations on a flow of Lennard-Jones liquids past a two dimensional circular cylinder. The ordinary boundary condition, i. e., the periodic boundary condition, gives significantly distorted velocity fields : for example, a flow near the boundary surfaces in the cylinder side direction flows along the boundary surfaces, namely, in the uniform flow direction ; the velocities near the boundary surfaces behind the cylinder are much larger compared to the results of the present boundary condition and Navier-Stokes equations ; a pair of vortices behind the circular cylinder are significantly distorted in the cylinder direction. On the other hand, the results of the present boundary condition agree well with those of Navier-Stokes equations qualitatively, and quantatively to a certain degree ; good agreement is obtained concerning the formation of a pair of vortices behind the circular cylinder and also concerning the flow fields in the side region of the cylinder. These results clearly show that the periodic-shell boundary condition is highly useful for non rigid molecular systems, and needless to say, for rigid molecular systems as well.

Reaction topology of excited state potential energy hypersurfaces

Topology ("rubber geometry") is an exceptionally suitable mathematical framework for a global quantum chemical description of the fundamental relations between nonrigid molecular systems, chemical reactions, and many photochemical processes. In Reaction Topology the concept of nuclear geometry is replaced by open subsets of an abstract nuclear configuration space nR. The ground and various excited state energy expectation value functionals induce a sequence of topologies in the nuclear configuration space nR, leading to a consistent topological description of molecular structure, excimers, exciplexes, and reaction mechanisms. Utilizing earlier results on topological properties of the abstract nuclear charge space wZ, a theorem is proven on the ordering of excited state potential energy hypersurfaces for sequences of isoelectronic molecules.