Quantum chemistry of nitrogen compounds. V. L.c.a.o.–m.o.–s.c.f. calculations on HCN using a basis set of Gaussian orbitals

Abstract

Linear combination atomic orbital – molecular orbital – self-consistent field (l.c.a.o.–m.o.–s.c.f.) calculations with a basis set of Gaussian orbitals have been applied to hydrogen cyanide. Basis sets ranging from 11 to 42 functions have been examined. The effect of variation of the exponent values for a 20 function basis set has been investigated and the "best" exponents have been employed in the larger sets. The basis set consisting of 39 Gaussian-type functions yielded an energy of −116.5760 atomic units (a.u.). Sigma orbital energies appear to be relatively insensitive to the size of the basis set. Properties such as first ionization potential, sigma–pi gap, and pi–pi* gap appear to be reasonably well reproduced with basis sets as small as 22, 26, and 28 basis functions. The internuclear distances calculated with the 39 function set are in good agreement with the experimental results. Examination of the coefficients of the pi orbitals on carbon and nitrogen reveals that the pi structure is very little perturbed by a change in the sigma core.