Improved virtual orbital equation

The most convenient procedure for attaining the minimum of the second order perturbation expression of the energy, so as to generate the optimized virtual orbitals, depends on the kind of problem being studied. In the case of intermolecular interactions, convergence is quite easy with just a gradient-based procedure. The minimization scheme can be recast in such a way that the coefficients of the improved virtual orbitals can be obtained, at each step, by a resolution of a linear system of NA+NB equations. Specifically. 

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