While several TEP values have been experimentally determined, it would be ideal if calculations could be used to rapidly predict accurate values for new phosphines since this information could be used for interpreting experiment results or as a basis for selecting a phosphine in the design of a new metal complex, especially in the spirit of Fey's approach. Density functional theory (DFT) calculations have been used to calculate TEP values. As one representative example, Clot used B3PW91 DFT calculations to calculate 68 CO frequency values for L-Ni(CO)3 complexes that included both phosphine and nonphosphine ligands. Importantly, this work established the ability to directly correlate and correct DFT calculated values with experiment. This work also demonstrated the transferability of calculated TEP values to other types of complexes, such as CpMn(CO)2L. Crabtree developed a bisphosphine scale that is related to TEP values. TEP values have also been interpreted using natural orbitals for chemical valence calculations, which allowed decomposition of donor and acceptor effects into specific types of orbital interactions. There have also been several studies showing fundamental property correlations with TEP values. For example, Koga used electrostatic potential values correlate with TEP values, and this indicates that relatively simple descriptors can provide understanding of donor and acceptor effects of phosphines.