The ability to perform ab initio electronic structure calculations that scale linearly with the system size is one of the central aims in theoretical chemistry. In this study, the implementation of the divide and conquer (DC) algorithm, an algorithm with the potential to aid the achievement of true linear scaling within Hartree-Fock (HF) theory, is revisited. Standard HF calculations solve the Roothaan-Hall equations for the whole system; in the DC-HF approach, the diagonalization of the Fock matrix is carried out on smaller subsystems. The DC algorithm for HF calculations was validated on polyglycines, polyalanines, and 11 real three-dimensional proteins of up to 608 atoms in this work. We also found that a fragment-based initial guess using the molecular fractionation with conjugated caps (MFCC) method significantly reduces the number of SCF cycles and even is capable of achieving convergence for some globular proteins where the simple superposition of atomic densities (SAD) initial guess fails.
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