See Linear Combination of Atomic Orbitals.

Linear Combination of Atomic Orbitals: An approximation which sums atomic orbitals (with certain coefficients) to produce molecular bonding and anti-bonding orbitals.

(Linear Combination of Atomic Orbitals) Molecular orbitals are usually constructed as a linear combination of atomic orbitals. The coefficients in this expansion are determined by solving the Schrödinger equation, typically in the Hartree-Fock approximation.