Abstract
The Hilbert space multireference coupled-cluster method with single and double excitations (HS-MRCCSD) is implemented in complete and special classes of incomplete model spaces. Vertical excitation energies of the water molecule are calculated. Good agreement (within 0.1 eV) is obtained with experimental values, and also with results of Fock space calculations which include effects of triple excitations in lowest order (FS-MRCCSD+T). The HS-MRCCSD functions are not pure spin states, and small energy differences occur between sublevels which should be degenerate.
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