An efficient linear scaling method for ab initio calculation of electron density of proteins

We present an efficient linear scaling method for direct calculation of electron density of proteins based on the molecular fractionation with conjugate caps (MFCC) approach. In the present approach, the MFCC method is first applied to computing electron densities of individual protein fragments and conjugate caps (concap). The total electron density of a protein molecule is obtained by summing over all individual electron densities of protein fragments subtracted by electron densities of concap molecules. Similar combination also directly yields electrostatic potential and dipole moment of protein from calculations of separate fragments. Detailed numerical tests on two peptides show that MFCC computed densities and dipole moments are in excellent agreement with those obtained from standard full system calculations at various ab initio levels.