Indian Journal of Chemistry Vol. 44A, November 2005, pp. 2195-2201 Electronic structure of seven component model protein chains: Effect of electron correlation Shyam Kishor Department of Chemistry, J.V. College, Baraut 250 611 , Uttar Pradesh, India and A K Bakhshi* Department of Chemistry, University of Delhi, Delhi 110007, India Email: akbakhshi2000@yahoo.com Received 26 April 2005; revised 14 September 2005 Quasi-particle band structure results of seven homopolypeptides with aliphatic side chains obtained using double zeta basis set have been used as input te calculate the electronic density of states of periodic and aperiodic model protein chains on the basis of negative factor counting method in ti ght binding approximation. The results obtained are compared to the electronic structure obtained from ab initio Hartree Fock band structure re sults for the same system, obtained using minimal basis set and double zeta basis set respectively. In our previous paperl, we have reported th e electronic structure of three-component model prote in chains using ab initio band structure results for polyglycine, polyalanine and poiyserine, obtained using minimal basis set. The effects of the change of the basis set (double zeta) and the secondary structure (a-helix) on the electronic density of states were also investigated. In continuation of our investigations into the electronic structure of protein s, in this paper we have extended our studies to seven- component system. We have studied the effect of electron correlation on the density of states of periodic and aperiodic seven-component polypeptide chain using a combination of the Moller Plesset many particle perturbation theory and the negative factor counting (NFC) method. Quasi-particle band structure results 2 of seven homopolypeptides with aliphatic side chains have been used as input to calculate th e electronic density of states of seven component polypeptide chains on the basis of negative counting method in tight binding approximation. The results obtained have been compared to the electronic structure results obtained using ab initio Hartree Fock band structure results 3 for the same system obtained using minimal basis set and double zeta basis set respectively. The geometry of the amino acid residues representing the elementary unit cell has been built up using standard bond length and bond angles. In the structure, the dihedral angle q> was -140° and the dihedral angle \f' was 140°(1). q>: tV: Methodology N 3 -C 4 -C 2 '-N3' (I) One of the principal deficiencies of the Hartree Fock crystal orbital method, is the neglect of correlation between motions of electrons with opposite spins. The result is that the Hartree Fock method overestimates the funr'.amental energy gap. Using band structures corrected for electron correlation we can obtain more realistic values. To determine the correlation corrected band structures of a periodic polymer, one has to perform first of all a Hartree Fock crystal orbital calculation.