Self-consistent field convergence for proteins: a comparison of full and localized-molecular-orbital schemes
Proteins in the gas phase present an extreme (and unrealistic) challenge for self-consistent-field iteration schemes because their ionized groups are very strong electron donors or acceptors, depending on their formal charge. This means that gas-phase proteins have a very small band gap but that their frontier orbitals are localized compared to “normal” conjugated semiconductors. The frontier orbitals are thus likely to be separated in space so that they are close to, but not quite, orthogonal during the SCF iterations. We report full SCF calculations using the massively parallel EMPIRE code and linear scaling localized-molecular-orbital (LMO) calculations using Mopac2009. The LMO procedure can lead to artificially over-polarized wavefunctions in gas-phase proteins. The full SCF iteration procedure can be very slow to converge because many cycles are needed to overcome the over-polarization by inductive charge shifts. Example molecules have been constructed to demonstrate this behavior. The two approaches give identical results if solvent effects are included.
KeywordsLinear scaling LMO-SCF NDDO Proteins Self-consistent field
This work was supported by the Deutsche Forschungsgemeinschaft as part of the Excellence Cluster Engineering of Advanced Materials and by the Bundesministerium für Bildung und Forschung as part of the high-performance Computer-Aided Drug Design (hpCADD) project.
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