A new module for constrained multi-fragment geometry optimization in internal coordinates implemented in the MOLCAS...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Molecular Modeling

Background:

• Optimizing relative positions and orientations of molecular fragments is crucial for understanding complex systems.
• Existing methods can be computationally expensive, especially for high-level electronic structure calculations.

Purpose of the Study:

• To implement and validate a parallel procedure for efficient geometry optimization of multiple molecular fragments.
• To develop a method that preserves the electronic structure of individual fragments during optimization.

Main Methods:

• A constrained fragment geometry optimization approach was implemented in the MOLCAS program package.
• The method utilizes internal "Z-matrix" coordinates and numerical derivatives for optimization.
• It handles both independent and covalently bonded fragments, scaling energy evaluations with the number of fragments, not atoms.

Main Results:

• The procedure demonstrated robust and smooth convergence to optimal structures in test calculations.
• It successfully applied high-level ab initio methods like CCSD(T) and CASPT2 to complex systems.
• Calculations on a benzene-lithium complex and an iron porphyrin-oxygen system validated the method's performance.

Conclusions:

• The developed constrained fragment geometry optimization is an efficient and accurate method for large molecular systems.
• Its ability to use high-level electronic structure methods makes it valuable for studying complex chemical interactions.