A pseudoatom approach to molecular truncation: application in ab initio MBPT methods.

Area of Science:

• Computational chemistry
• Quantum chemistry

Background:

• The molecular truncation method by Mallik et al. was initially developed for semiempirical Neglect of Diatomic-Mulliken-Overlap (NDDO) methods.
• Applying this method to higher-level ab initio methods like Møller–Plesset perturbation theory (MBPT) requires validation.

Purpose of the Study:

• To evaluate the performance of the molecular truncation method using pseudoatoms within ab initio MBPT frameworks.
• To assess the accuracy of pseudoatom representations for specific functional groups (-OCH(3) and -OCH(2)CH(3)) in molecular geometry optimizations.

Main Methods:

• Utilized pseudoatoms, comprising parametrized effective core potentials and external charge fields, to replace functional groups in molecular clusters.
• Performed geometry optimizations on pseudoatom-terminated clusters using second-order Møller–Plesset perturbation theory (MP2) and coupled-cluster with singles and doubles (CCSD) levels of theory.
• Compared the optimized geometries and deprotonation energies of pseudoatom-terminated clusters with those of full, all-atom molecules.

Main Results:

• The pseudoatom method demonstrated good performance at both MP2 and CCSD levels of theory.
• Optimized geometries and calculated deprotonation energies for pseudoatom-terminated clusters were comparable to full molecules.
• Significant reductions in computational time were observed for pseudoatom-terminated clusters.

Conclusions:

• The molecular truncation method with pseudoatoms is effective for ab initio calculations at MP2 and CCSD levels.
• This approach offers a computationally efficient alternative to full molecular calculations without substantial loss of accuracy for the studied systems.