Quantum Monte Carlo simulation of intermolecular excited vibrational states in the cage water hexamer

Abstract

Articles you may be interested inOn the ground state calculation of a many-body system using a self-consistent basis and quasi-Monte Carlo: An application to water hexamer A new ab initio intermolecular potential energy surface and predicted rotational spectra of the Kr−H2O complex J. Chem. Phys. 137, 224314 (2012); 10.1063/1.4770263 Communication: Quasiclassical trajectory calculations of correlated product-state distributions for the dissociation of (H2O)2 and (D2O)2 J. Chem. Phys. 135, 151102 (2011); 10.1063/1.3655564Elucidating the role of many-body forces in liquid water. I. Simulations of water clusters on the VRT(ASP-W) potential surfaces Rigid-body diffusion Monte Carlo simulations of the ground state and ten low-lying intermolecular excited vibrational states for the cage form of (H 2 O) 6 are reported. The excited states are found by a nodal optimization procedure in which the fundamental excited-state nodes are constructed from the harmonic normal coordinates. The anharmonic effects in the excited states are found to be large. One of the states with relatively large transition intensity involves primarily flipping motions of the free OH bonds on the doubly bound monomers, and is assigned to the vibration-rotation-tunnelling band observed experimentally by Liu et al. ͓Nature 301, 501-503 ͑1996͔͒.

Accurate Energies and Structures for Large Water Clusters Using the X3LYP Hybrid Density Functional

“…31 In addition, methods using a nuclear QMC scheme to calculate zero-point effects have used different model potentials. 44,45 Experiments have also been ambiguous, with cage structures observed in water clusters formed from supersonic jets 33 and cyclic structures observed in clusters formed in liquid helium droplets 38,46 or solid para-hydrogen matrices. 39 Our results using X3LYP/aug-cc-pVTZ(-f) indicate that the book and cyclic (chair) structures are the most stable (Table 3, Figure 4).…”

Importance sampling in rigid body diffusion Monte Carlo

“…The rigid body rotations are carried out using either Euler angles, or quaternion algebra [16]. Considerable use has been made of this algorithm for water clusters, where the floppy intermolecular degrees of freedom are difficult to study with other methods for clusters larger than the water dimer [1,2,4,6,7,10,11,[14][15][16].…”

“…A convenient Gaussian short-time approximation for such monomer rotational motion was assumed in that work. A number of applications of RBDMC have since been carried out, all of which use a combination of this angular sampling for monomer rotation in local monomer principal axis frames, together with the conventional sampling of translational motions of monomers in the laboratory frame [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. The rigid body rotations are carried out using either Euler angles, or quaternion algebra [16].…”

“…More efficient sampling of excited states within fixed node is often possible by using explicit trial functional forms. Exploration of nodal release, nodal optimization, spectral evolution and other approaches to go beyond the fixed node approximation is an active area of research [10,[24][25][26][27][28][29][30][31][32][33][34].…”

On the fly nodal searches in importance sampled fixed-node diffusion Monte Carlo using a parallel, fine-grained, genetic algorithm

“…Obviously, this is a difficult problem because one must know in advance the nodal topology of the unknown state being sought. For this reason, finding nodal hypersurfaces of many-body wave functions is an active area of investigation [7,[20][21][22][23][24][27][28][29][30][31].…”