Perturbation approach to constrained electron transfer in density functional theory

As an alternative to the variational constrained density functional theory a simpler perturbative approach based on the Taylor series expansion, around the ground state of the system, of the density matrix and the electronic density, as functions of the Lagrange multiplier, introduced to impose the constraint in the variational procedure, is developed and implemented in deMon2k. The first and second order corrections introduced are evaluated with the information contained in the ground state calculation, through the use of auxiliary density perturbation theory, while the value of the Lagrange multiplier at which the series is evaluated is determined from the amount of charge transferred. The energy differences between the ground and the constrained states obtained with the first and second order perturbative terms lie, in general, close to the variationally determined values. In addition, it is found that the results are practically independent of the exchange–correlation energy functional used and show rather small variations with respect to the basis set.