Hartree-Fock quality wave function

In this research, the coupled-cluster single and double substitutions method with a perturbative treatment of triple excitations CCSD(T) [16-18] was adopted based on unrestricted Hartree-Fock reference wave functions. For the CCSD(T) computations, the cc-pVDZ, cc-pVTZ, and cc-pVQZ quality basis sets were used, where cc-pVnZ is an abbreviation for the correlation-consistent polarized valence basis sets of Dunning et al. [19-22]. With the coupled-cluster methods used in this research, the core orbitals are frozen. That is, the Is-like molecular orbital is frozen for O, while the Is2s2p3s3p3d-like molecular orbitals are frozen for Br. 

Other calculations tested using this molecule include two-dimensional, fully numerical solutions of the molecular Dirac equation and LCAO Hartree-Fock-Slater wave functions [6,7] local density approximations to the moment of momentum with Hartree-Fock-Roothaan wave functions [8] and the effect on bond formation in momentum space [9]. Also available are the effects of information theory basis set quality on LCAO-SCF-MO calculations [10,11] density function theory applied to Hartree-Fock wave functions [11] higher-order energies in... 

In our present work, V(r) is in most instances determined from an ih initio STO-5G SCF molecular orbital wave function, using the GAUSSIAN programs [18-20], with all integrals in equation (1) evaluated rigorously. It has been shown that electrostatic potentials obtained even from SCF wave functions that are not near Hartree-Fock quality are generally reliable [33,44-49]. Many alternative procedures for computing V(r) have been discussed and analyzed in depth elsewhere [35,36,50]. 

Just as in the unrestricted Hartree-Fock variant, the Slater determinant constructed from the KS orbitals originating from a spin unrestricted exchange-correlation functional is not a spin eigenfunction. Frequently, the resulting (S2) expectation value is used as a probe for the quality of the UKS scheme, similar to what is usually done within UHF. However, we must be careful not to overstress the apparent parallelism between unrestricted Kohn-Sham and Hartree-Fock in the latter, the Slater determinant is in fact the approximate wave function used. The stronger its spin contamination, the more questionable it certainly gets. In... 

Despite these restrictions, the GVB and SC methods generally provide energies that are much closer in quality to CASSCF than to Hartree Fock (19), and wave functions that are close to the CASSCF wave function having the same number of electrons and orbitals in the active space. This property has been used to devise a fast method to get approximate SC wave functions. 

When comparing STO and CGTF basis sets of a particular size, it should be taken into account that with STO basis sets the results are only due to exponents of STO s, whereas with the CGTF basis sets also the effects of the number of primitives and their contractions are involved. Furthermore it should be kept in mind that the total energy is a rather insensitive test of the quality of wave functions, so that also some other molecular properties should be considered. Finally, a rigorous comparison should also include calculations going beyond the Hartree-Fock limit. 

The ADMA and SADMA methods generate ab initio quality density matrices P for large molecules M, while avoiding the computation of macromolecular wave functions. At the Hartree-Fock level, the first-order density matrix P fully determines all higher-order density matrices. Within the Hartree-Fock framework, expectation values for one-electron and two-electron operators can be computed using the first-order and second-order density matrices. Consequently, the ADMA and SADMA methods provide new possibilities for adapting quantum-chemical techniques for macromolecules. 

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