wavefunctionmetod

Wavefunction method is a class of computational approaches in quantum chemistry and physics that aim to determine the quantum state of a many-electron system by solving for its wavefunction Ψ rather than solely its electron density. Under the Born–Oppenheimer approximation, these methods solve the electronic Schrödinger equation HΨ = EΨ, where H includes kinetic energy and electron–electron repulsion. A central idea is to express Ψ as a function of the coordinates and spins of all electrons and to approximate it by a finite basis set, often built from atomic orbitals. The variational principle ensures that computed energies are upper bounds to the true ground-state energy.

Hartree–Fock theory provides a mean-field reference wavefunction, represented as a single Slater determinant, and captures exchange but not dynamic correlation. Post-Hartree–Fock methods aim to recover electron correlation by considering excited configurations: configuration interaction (CI), coupled-cluster (CC) methods, multi-reference CI (MRCI), and complete active space (CAS) approaches. Full CI is exact within the given basis but computationally prohibitive for all but small systems; practical post-HF methods scale steeply with system size.

Wavefunction methods are widely used for high-accuracy thermochemistry, spectroscopy, reaction energies, and excited states (e.g., EOM-CC, CAS-based methods). Advantages include explicit treatment of electron correlation and systematic improvability; limitations include high computational cost, limited scalability to large systems, and sensitivity to basis-set quality. They stand in contrast to density functional theory, which emphasizes electron density rather than the many-electron wavefunction; DFT is generally more scalable but offers less systematic control over accuracy.