RHF methodology

RHF methodology, used to investigate the cycloaddition reaction of isocyanic acid with methylenimine, confirmed a two-step mechanism via a cis intermediate for the equimolar reaction.22 The 2 + 2-cycloaddition of chlorosulfonyl and trichloroacetyl... 

In the HF scheme, the first origin of the correlation between electrons of antiparallel spins comes from the restriction that they are forced to occupy the same orbital (RHF scheme) and thus some of the same location in space. A simple way of taking into account the basic effects of the electronic correlation is to release the constraint of double occupation (UHF scheme = Unrestricted HF) and so use Different Orbitals for Different Spins (DODS scheme which is the European way of calling UHF). In this methodology, electrons with antiparallel spins are not found to doubly occupy the same orbital so that, in principle, they are not forced to coexist in the same spatial region as is the case in usual RHF wave functions. 

The examples included in this and the subsequent sections include the normal alkanes C H2 +2, with n = 12, 16, 20, 24, 28, and 32, and the (H20) clusters with n = 10, 12, 14, 16, and 20. All the CC calculations discussed in this chapter employ the RHF determinant as a reference and freeze the core orbitals correlating with the Is orbitals of the C and O atoms. In each case, we use the six Cartesian components of the d orbitals present in the AO basis sets used in our calculations. In this section, we focus on the CIM-CC calculations performed with the 6-3 lG(d) and 6-31G(d, p) basis sets [97, 98]. The CIM-CCSD calculations for (H20) clusters with the larger 6-3H-- -G(d, p) [97-99] and 6-31 H--1-G(d, p) [99, 100] basis sets, which contain diffuse functions and create an interesting as well as challenging situation for the CIM methodology, are discussed in the next section. 

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