The bond energy is the sum of N_bond terms where each term has the form:

kbond ( bij - bo )2

where k_bond is a force constant, b_ij the current length of the bond between atoms atom_i and atom_j and b_o is the equilibrium bond length. (The bond energy function should properly include a one half factor; therefore, what we consider to be the force constant is actually half the value of a proper force constant.) The bond record in the descriptor file has the form

bond	Nbond
	atomi	atomj	kbond	bo
	...	...	...	...

The bond record starts with the keyword bond followed by the number of bonds N_bond, then N_bond lines of four fields each: the atom numbers of the pair of atoms that make up a bond atom_i and atom_j, the bond force constant k_bond and the equilibrium bond length b_o. Example

bond   3

1   2   100326.1   1.54
2   3   100326.1   1.54
3   4   100326.1   1.54

A bond record containing three bonds. The first bond links atoms 1 and 2, the second links atoms 2 and 3 and the last links atoms 3 and 4. Each bond has the same force constant 100326.1 CEU Å^-2 (240 kcal/mol/Ų) and the same equilibrium bond length 1.54Å.