Grid-Adapted-Manybody-Analysis (GAMA): A Fragment-Based Quantum Chemistry Exploration on Medium-Sized Water Clusters and the Critical Role of Counterpoise Correction

Performing ab initio calculations on large molecular systems is challenging. Fragment-based quantum chemistry (FBQC) provides an efficient alternative. We introduce GAMA(Grid-Adapted-Manybody Analysis), a many-body based expansion (MBE) method that fragments a molecular system into grid-based monomers. Additionally, GAMA employs an ONIOM-style multilevel framework to capture long-range interactions. We applied GAMA to water clusters, demonstrating that increasing monomer size improves accuracy in reproducing unfragmented MP2 energies across various basis sets. Counterpoise correction was found essential for mitigating Basis Set Superposition Error (BSSE) and ensuring accuracy. Compared to direct MP2 calculations, GAMA significantly reduces computational cost while maintaining high accuracy. These results highlight GAMA's potential for accurate and efficient high-level quantum chemistry calculations on water clusters towards enabling quantum mechanics for large molecular systems.