1. Weinhold, F. and Landis, C. R.Valency and Bonding: A Natural Bond Orbital Donor-Acceptor Perspective;Cambridge University Press:Cambridge U.K., 2005; p281ff.
2. Kittel, C.Introduction to Solid State Physics,7th ed.Wiley:New York, 1995, p161ff.Mott, N. F. and Jones, H.The Theory of the Properties of Metals and Alloys;Dover:New York, 1958.
3. Natural bond orbital methods