Excited states of van der Waals clusters by projector Monte Carlo, with application to excitations of molecules in small He-4(n)

We demonstrate the extraction of excited rovibrational state energies for q uantum systems with up to 15 dimensions, using the recently developed proje ction operator imaginary time spectral evolution (POITSE) method. The techn ique is applied to a series of model van der Waals complexes of the form AB -X-n, n = 1-3, with heavy AB and a weakly bound, light atom X. The selectiv ity of several types of molecule-fixed and space-fixed projectors and their relationship to the nature of the accessible excited states are studied by making comparison with discrete variable-finite basis representation (DVR- FBR) calculations for n=1. These also allow identification of excitations d ominant in ground state infrared absorptions. The results suggest a propens ity for space-fixed projectors to access states where the angular momentum of the diatomic AB dominates the overall angular momentum, while molecule-f ixed projectors access primarily bending and stretching modes. The relevanc e of this observation for the interpretation of the rovibrational spectra o f molecules embedded in large liquid helium clusters is discussed. (C) 1999 American Institute of Physics. [S0021-9606(99)01912-1].