CASMP2, CASPT2 methods

Canonical momentum operator, 248 Car and Parrinello (CP) method, 388 CASMP2, CASPT2 methods, 132 CASVB method, 202 Cavity, in reaction field models, 393 CBS-4, CBS-q, CBS-Q and CBS-APNO methods, 167... 

CASPT2 (CASMP2) A method of calculation involving second-order perturbation theory done after a CASSCF calculation. 

Just as single reference Cl can be extended to MRCI, it is also possible to use perturbation methods with a multi-detenninant reference wave function. Formulating MR-MBPT methods, however, is not straightforward. The main problem here is similar to that of ROMP methods, the choice of the unperturbed Hamilton operator. Several different choices are possible, which will give different answers when the tlieory is carried out only to low order. Nevertheless, there are now several different implementations of MP2 type expansions based on a CASSCF reference, denoted CASMP2 or CASPT2. Experience of their performance is still somewhat limited. 

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