Combined diffusion quantum Monte Carlo-vibrational self-consistent field (DQMC-VSCF) method for excited vibrational states of large polyatomic systems

Abstract

Abstract Self-consistent field (SCF) solutions of the Schrodinger equation can provide approximate, often very accurate, nodes for DQMC calculations of electronic structure. As shown in this paper, SCF calculations can do the same in specifying nodes for vibrational states of polyatomic systems as large as Ar13. The vibrational selfconsistent field (VSCF) methoda is approximate but quite accurate in itself, and it has been used previously for prediction of low-level excitations of protein molecules, water clusters, and other complex systems. In the calculations described in this paper for Ar3 and Ar13, the SCF function was specified as a product of functions for each of the normal modes, and solutions were obtained with a sixth-order expansion reducing the required integrals to one dimension. For the argon clusters, the potential energy was specified as additive pair potentials. The DQMC calculations were performed using the VSCF nodes as absorbing boundaries without the use of the wave function for importance sampling. In the case of Ar3, the DQMC-VSCF results could be compared with nearly exact results from other methods, and they were found better than those of VSCF alone but with errors as high as 8%. In the case of Ar13, the excitation energies for DQMC-VSCF and VSCF were found quite close to each other for the 33 fundamental excited states, thus exhibiting small “correlation” effects. It was noted that the combined method is better than the VSCF method and that improving on VSCF as with a vibrational M ller Plesset theory might provide even better results.