Figure : A schematic of how dummy atoms are added to near-linear angles. An angle α abc is considered near-linear when it is above 165°. So long as α abc is not perfectly linear, the dummy atom (x) is added in a direction perpendicular to the plane defined by the angle's three atoms. The bond stretch coordinate δ bx is constrained to 1 Å and the two bending angles α abx and α cbx are constrained to 90°. The nearly-linear angle is replaced by the improper dihedral τ abxc .