ON THE CALCULATION OF 1ST-ORDER PROPERTIES - EXPECTATION VALUE VERSUS ENERGY DERIVATIVE APPROACH

The definition of so-called first-order properties is, in general, not unique for approximate solutions of the Schrodinger equation. In the present work, the determination of molecular properties as the expectation value or energy derivative are compared for a wide class of electron correlation treatments that are based on the stationarity of certain modified energy functionals. The case in which all perturbation-dependent parameters the energy expression are optimized is taken as a reference point, to which the derivative and the expectation value approaches are related by a sequence of approximations. By analyzing the validity of these approximations, we are in a position to give general criteria for the choice of the method to be employed. The derivative approach is shown to be a special case of a general method to improve the calculation of molecular properties. Furthermore, this method is extended to give a correction to the energy itself.