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

Diffusion quantum Monte Carlo (DQMC) is a powerful method for calculating the vibrational ground state of large systems but is inapplicable in general for excited states. We propose a general method for excited states, based on combining DQMC with the approximate vibrational self-consistent field (VSCF) approach; the latter is used to obtain the nodes of the excited state wavefunctions. The combined DQMC–VSCF is approximate but found high accuracy in test calculations of Ar3. DQMC–VSCF is also applied to collective mode excitations of Ar13. The method provides full spectroscopic assignment for the computed excited states through VSCF.