Ab Initio Calculation
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Although the term ab initio usually indicates calculations based on first principles, either they are related to classical or to quantum mechanics, this term is generally used when it refers to calculations based on quantum mechanics first principles (Szabo and Ostlund 1994). Thus, if the d’Alembert equation is used to describe a force balance, in which the expression of the forces is only expression of chemical-physical average properties (values obtained as average of microscopic properties) without any use of empirical adjustable parameters, the calculation can also be defined ab initio. As mentioned, in general, the term ab initio is used when the calculations are performed by using the Schrödinger equation or rather an eigenvalue equation for the quantum system. However, attention must be paid to the quantities or potential operators that define the eigenvalue equations. In fact, if the operators in the Hamiltonians are defined using experimental data or by means of fitting...
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