STO-1G “Basis Set

Having specified a Hartree-Fock calculation on singlet HHe+, with H—He = 0.800 A (1.5117 bohr), using an STO-1G basis set, the most straightforward way to proceed is to now calculate all the integrals, and the orthogonalizing matrix S 1/2... 

Table 5.1 Results of initial guess and SCF cycles on HHe+ at bond length 0.800 A using the STO-1G basis set. Energies (ej, e2, and E f1) at in hartrees...

Table 2 Computed geometry, energies, and dipole moments (p) of phosphalkenes with ST0-3G and STO 43-1G basis sets...

As we just saw, MP2 calculations utilize the Hartree-Fock MOs (their coefficients c and energies e). The HF method gives the best occupied MOs obtainable from a given basis set and a one-determinant total wavefunction i(i, but it does not optimize the virtual orbitals (after all, in the HF procedure we start with a determinant consisting of only the occupied MOs - Sections 5.2.3.1-5.2.3.4). To get a reasonable description of the virtual orbitals and to obtain a reasonable number of them into which to promote electrons, we need a basis set that is not too small. The use of the STO-1G basis in the above example was purely illustrative the smallest basis set generally considered acceptable for correlated calculations is the 6-31G, and in fact this is perhaps the one most frequently used for MP2 calculations. The 6-311G basis set is also widely used for MP2 and MP4 calculations. Both bases... 

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