Atomic charges from IR intensity parameters: theory, implementation and application

A computational method to extract atomic charges (IR charges) from DFT/ab initio-computed atomic polar tensors is presented and compared with commonly available population schemes. The procedure adopted and its implementation in a (freely available) code are also reported and commented. Thanks to the procedure developed, infrared charges can be now readily gathered also by non-experts, provided that Cartesian dipole derivatives from a quantum chemical calculation of the IR spectrum are available. The method has been applied for the calculation of IR charges of about 50 molecules: It performs well in describing peculiar intramolecular interactions, providing a picture of the molecular charge distribution coherent with the chemical intuition. A nice agreement is also found with charges obtained from Hirshfeld algorithm and from the fitting of the electrostatic potential (CHELPG and MK schemes). On this basis, we propose IR charges as a reliable, physically sound and easily accessible alternative to other charge parameters currently adopted.