HPAM: Hirshfeld Partitioned Atomic Multipoles.

Area of Science:

• Computational chemistry
• Quantum chemistry
• Theoretical chemistry

Background:

• Accurate atomic charge and multipole calculations are crucial for understanding molecular electrostatic properties.
• Existing methods like Hirshfeld (HD) provide foundational approaches to charge partitioning.
• Limitations in reproducing electrostatic properties necessitate improved partitioning schemes.

Purpose of the Study:

• To implement and evaluate the Hirshfeld-Iterated (HD-I) atomic charge density partitioning scheme.
• To assess the accuracy of HD and HD-I methods for calculating atomic charges and multipoles.
• To compare the performance of HD and HD-I against ab initio calculations for molecular electrostatic properties.

Main Methods:

• Implementation of Hirshfeld (HD) and Hirshfeld-Iterated (HD-I) atomic charge density partitioning.
• Calculation of atomic charges and multipoles up to l(max)=4 (hexadecapoles).
• Comparison of calculated molecular multipole moments and electrostatic potentials (ESP) with reference ab initio data.

Main Results:

• HD-I generally shows superior reproduction of ab initio electrostatic properties compared to HD.
• Increasing the atomic multipole rank (l(max)) systematically enhances precision.
• Atomic multipoles up to rank l(max) exactly reproduce ab initio molecular multipole moments for L ≤ l(max).
• Using only atomic charges (l(max)=0) for HD or HD-I results in significant errors in molecular dipole moments.

Conclusions:

• The HD-I method offers improved accuracy for atomic charge and multipole calculations.
• Higher-rank multipoles are essential for precise electrostatic property reproduction.
• Simple atomic charges from HD or HD-I alone are insufficient for accurate molecular dipole moment calculations.