Free valence

Definition:

V_max - maximum valence of atom A

P_A - total electronic population on atom A

References:

1. I.G. Csizmadia, Theory and Practice of MO Calculations on Organic Molecules, Elsevier, Amsterdam, 1976.
2. A.B. Sanniraghi, Adv. Quant. Chem., 1992, 23, 301-351.