AB INITIO AND DENSITY-FUNCTIONAL TREATMENTS OF MOLECULES

The four main approaches to calculating molecular properties are ab initio methods, semiempirical methods, the density-functional method, and the molecular-mechanics method. [Pg.480]

Semiempirical molecular quantum-mechanical methods use a simpler Hamiltonian than the correct molecular Hamiltonian and use parameters whose values are adjusted to fit experimental data or the results of ab initio calculations an example is the Hiickel MO treatment of conjugated hydrocarbons (Section 16.3), which uses a one-electron Hamiltonian and takes the bond integrals as adjustable parameters rather than quantities to be calculated theoretically. In contrast, an ab initio (or first principles) calculation uses the correct Hamiltonian and does not use experimental data other than the values of the fundamental physical constants. A Hartree-Fock SCF calculation seeks the antisymmetrized product d of one-electron functions that minimizes / dr, where H is the true Hamiltonian, and is thus an ab initio calcula- [Pg.480]