Bonding : A Natural Bond Orbital Donor - Acceptor Perspective

Exact answers to practically all chemical questions are, in principle, obtainable from solutions of this equation.3 Thus, Eq. (1.1) is the ultimate oracle of chemical knowledge. Equation (1.1) contains threemathematical entities: (i) theHamiltonian operator Ĥ , determined by the choice of chemical system; (ii) thewavefunctionΨ, describing the allowed spatial distribution of electrons and nuclei of the system; and (iii) the energy level E associated with Ψ. The Hamiltonian Ĥ contains terms representing kineticand potential-energy contributions, depending only on fixed properties (e.g., mass, charge) of the electrons and nuclei that compose the chosen system of interest. Hence, Ĥ is the “known” and Ψ and E are the “unknowns” of Eq. (1.1). Mathematically, Ĥ is an operator that modifies the wavefunction Ψ( r, R) appearing on its right, where we write r = ( r1, r2, . . ., rN ) to denote the collective coordinates of N electrons and R = ( R1, R2, . . ., Rν) those of ν nuclei. Only for