## Abstract

According to the spin-coupled theory of structure of molecules, the wavefunction is constructed from configurations of non-orthogonal orbitals whose spins are coupled to the required overall resultant in all allowable ways (Gerratt 1971). This approach is here applied to the molecules LiH, BH, Li$_{2}$ and HF. Two cases are considered: (i) a single configuration of non-orthogonal orbitals and (ii) a linear combination of two such configurations. The resulting wavefunctions are used to calculate theoretical values for a series of molecular properties including binding energy, equilibrium internuclear distance, force constant, dipole (or quadrupole) moment, dipole (or quadrupole) moment derivative, electric field gradient at a nucleus and net force on a nucleus. The values of these properties, particularly in case (ii), are all in excellent agreement with experiment. Notably, the calculated values of binding energies are 90-96% of observed, except for Li$_{2}$ where the corresponding percentage is 80. The quality of these wavefunctions is considerably better than many-structure v.b. functions, and comparable with very large m.o.-c.i. or m.c.s.c.f. wave functions. The implementation of this approach involves the use of highly efficient methods for calculating the necessary $n$-electron density matrices, and for optimizing the orbital and spin-coupling coefficients. These techniques are fully described.