Comparison of fully internally and strongly contracted multireference configuration interaction procedures

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Electronic Structure Theory

Background:

• Multireference (MR) methods are crucial for systems with strong electron correlation, like transition metal complexes.
• Traditional MR methods face computational challenges due to the explosive growth of configuration state functions (CSFs) with system size.
• Internal contraction schemes (FIC, SC) reduce computational cost by limiting the number of variational parameters.

Purpose of the Study:

• To implement and benchmark fully internally contracted (FIC-MRCI) and strong contraction (SC-MRCI) methodologies.
• To compare the accuracy and efficiency of FIC-MRCI and SC-MRCI against traditional uncontracted MRCI and partially internally contracted (PC-MRCI).
• To evaluate the performance of these methods for ground and excited states, and potential energy surfaces of small molecules.

Main Methods:

• Implementation of FIC-MRCI and SC-MRCI using a computer-assisted strategy.
• Benchmarking against uncontracted MRCI and PC-MRCI for various molecular systems (N2, O2, CO, etc.).
• Investigation of potential energy surfaces for representative chemical transformations.

Main Results:

• FIC-MRCI and PC-MRCI exhibit comparable accuracy and efficiency, with average errors of 2-6% relative to uncontracted MRCI correlation energies.
• Excitation energies are reproduced with an accuracy of approximately 0.2 eV, even for complex electronic transitions in transition metal complexes.
• The SC-MRCI scheme demonstrates no computational advantage and yields significantly larger errors compared to PC-MRCI and FIC-MRCI.

Conclusions:

• FIC-MRCI and PC-MRCI are accurate and efficient alternatives to traditional MRCI for studying electronic structure.
• The SC-MRCI scheme is not recommended due to its computational inefficiency and reduced accuracy.
• The implemented FIC-MRCI and PC-MRCI methods provide reliable tools for quantum chemistry calculations, particularly for challenging systems.