1. Löwdin P-O (1954) Q Prog Rep Solid-State Mol Theor Group MIT 4: 1
2. a) Löwdin P-O (1955) Phys Rev 97: 1474; (b) Löwdin P-O (1955) Phys Rev 97: 1490; (c) Löwdin P-O (1955) Phys Rev 97: 1509
3. McWeeny R (1959) Proc R Soc Lond Ser A 253: 242
4. McWeeny R (1960) Rev Mod Phys 32: 335
5. McWeeny R (1955) Tech Rep Solid-State Mol Theor Group MIT 7: 1