Wave function double perturbation expansion

A general approach to the intramonomer correlation problem is known as the many-electron (or many-body) SAPT method88,141 213-215. In this method the zeroth-order Hamiltonian H0 is decomposed as H0 = F + W, where F = FA + FB is the sum of the Fock operators, FA and FB, of monomer A and B, respectively, and W is the intramonomer correlation operator. The correlation operator can be written as W = WA + WB, where Wx = Hx — Fx, X = A or B. The total Hamiltonian can be now be represented as H = F + V + W. This partitioning of H defines a double perturbation expansion of the wave function and interaction energy. In the SRS theory the wave function is obtained by expanding the parametrized Schrodinger equation as a power series in and A,... 

If the R-ELHAV expansion is able to effectively reproduce the part of the interaction energy missing in Sr-srs in a low-order treatment (as will be seen in Sect. 4, this is the case), it is desirable to extend this theory to obtain a perturbation expansion that starts from 0o and takes both Vp and Vt into account. For this purpose, the most straightforward idea is to develop some double perturbation expansion in Vp and Vt which treats these two perturbations in an SRS-like and ELHAV-like way, respectively. The formulae defining the wave function corrections in this double perturbation theory can be obtained by expanding the equation... 

All the calculations are performed in momentum space and (unless otherwise stated) plane waves with kinetic energy up to 9.15 Ry are included in the expansions of the wave functions. Only those with kinetic energy S 2.55 Ry are dealt with exact, the remaining ones are treated by Lowdin perturbation theory up to second order. This corresponds to approximately 21 + 125 waves when working with the two-atoms cells, 43 + 240 when working with the doubled (four atoms) ui it cells, 85 + 500 on quadrupled cells, etc. Two to five special k-points are used for Brillouin zone integration (corresponding to (222) in the notation of... 

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