Determining atom‐centered monopoles from molecular electrostatic potentials. The need for high sampling density in formamide conformational analysis

TLDR

The study presents an improved CHELPG method for computing potential‑derived charges based on the CHELP program. The authors implemented the CHELPG point‑selection algorithm, sampling an equally spaced grid of points every 0.3 Å (excluding points >2.8 Å or within van der Waals radii) to analyze charge changes during formamide rotation, and compared the results to the original CHELP program. CHELPG achieved considerably less orientation dependence and better rotational invariance than CHELP, whereas the original CHELP program failed to provide satisfactory invariance even with thousands of points.

Abstract

Abstract An improved method for computing potential‐derived charges is described which is based upon the CHELP program available from QCPE. 1 This approach (CHELPG) is shown to be considerably less dependent upon molecular orientation than the original CHELP program. In the second part of this work, the CHELPG point selection algorithm was used to analyze the changes in the potential‐derived charges in formamide during rotation about the CN bond. In order to achieve a level of rotational invariance less than 10% of the magnitude of the electronic effects studied, an equally‐spaced array of points 0.3 Å apart was required. Points found to be greater than 2.8 Å from any nucleus were eliminated, along with all points contained within the defined VDW distances from each of the atoms. The results are compared to those obtained by using CHELP. Even when large numbers of points (ca. 3000) were sampled using the CHELP selection routine, the results did not indicate a satisfactory level of rotatational invariance. On the basis of these results, the original CHELP program was found to be inadequate for analyzing internal rotations.