BLYP

BLYP is a widely used density functional in quantum chemistry that belongs to the class of generalized gradient approximations (GGAs) in Kohn–Sham density functional theory. The name BLYP comes from the combination of Becke’s exchange functional (Becke 1988, B88) with the Lee–Yang–Parr correlation functional (LYP, 1988). As a pure functional, BLYP does not include any portion of exact exchange from Hartree–Fock theory.

In practical terms, BLYP uses the gradient-corrected exchange energy density from Becke and a correlation energy density developed by Lee, Yang, and Parr. This pairing was proposed to improve upon local density approximations by incorporating information about the density gradient, which helps describe inhomogeneous electron systems such as molecules.

BLYP has been popular for calculating molecular geometries, vibrational frequencies, and relative energies in organic and biochemical systems. It generally provides reliable geometries and reasonable energetics for many systems, but it has notable limitations. It tends to underestimate reaction barriers and can underbind certain species, especially where dispersion forces play a significant role. Like many GGAs, it also suffers from the lack of long-range dispersion accuracy and from self-interaction errors, which can affect certain properties and systems, such as transition metals or strongly correlated cases. To address dispersion, practitioners often use dispersion-corrected variants (for example, BLYP-D3).

The functional is implemented in major quantum chemistry software packages and is commonly used in conjunction with various basis sets. While some modern functionals (e.g., PBE, B3LYP) have become more popular in certain domains, BLYP remains a foundational option for molecular studies and method benchmarking.