Massively parallel computers allow the application of highly accurate wave-function methods to smaller molecules of biological interest.
Such methods consist of an increment scheme, i.e. computing energies in consistent basis sets of increasing size on a low correlation level (i.e. Moller-Plesset Perturbation Theory in second order) extrapolating the result to the complete basis set, and finally correcting it by using a highly-correlated method (e.g. coupled-cluster singles, doubles and corrected triples, CCSD(T)). Near degeneracies of orbitals, e.g. when the chemical bonds are broken, requires the usage of multi-reference methods.