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.

01 Sep 2004·Journal of Physical and Chemical Reference Data

Use of Semiempirical Methods for Detecting Anomalies in Reported Enthalpies of Formation of Organic Compounds

Author: Stewart, James J. P.

Using semiempirical methods, a survey of the enthalpies of formation of ∼1300 compounds in the NIST Chemistry WebBook database was performed and compared to the reported values. Five sets of compounds were identified: a large set in which theory and experiment agreed, a set in which they disagreed strongly due to transcription errors, a set in which they disagreed strongly due to experimental errors, a set that disagreed strongly due to errors in the method, and a set in which the difference between calculation and experiment was not obvious.

01 Jan 1999·Crystal Engineering The Design and Application of Functional Solids

Developing a Semiempirical Method

Author: Stewart, James J. P. ; Stewart, Anna C.

The philosophical considerations involved in the development of a new semiempirical method are discussed.Although increased accuracy is the most important desideratum, other features are also of great importance.Among these are the use to which the method will be put, the theor. validity of the approximations, and the numerical stability of the method.

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