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.