The angl angle energy is the sum of N_angle terms where each term has the form:

kangl ( qijk - qo )2

where k_angl is a force constant, q_ijk the current angle spanned by atoms atom_i, atom_j and atom_k and q_o is the equilibrium angle. (The angle 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 angl record in the descriptor file has the form

angl	Nangle
	atomi	atomj	atomk	kangl	qo
	...	...	...	...	...

The angl record starts with the keyword angl followed by the number of angles N_angle, then N_angle lines of five fields each: the three atoms that span an angle atom_i, atom_j and atom_k, the angle force constant k_angl and the equilibrium value of that angle q_o. Example

angl   2

81   84   82   18268.3   1.61792
84   87   85   18268.3   1.42419

An angl record containing two angles. The first angle is spanned by atoms 81, 84 and 82 and the second angle is spanned by atoms 84, 87 and 85. Both angles have the same force constant of 18268.3 CEU radian^-2 (43.7 kcal/mol/radian²). The first angle has an equilibrium angle of 1.61792 radians (92.7 degrees).