a40 - Angle for phosphorous

a40 is another parameter along with sulfonamides (reported before) that keeps on moving to non-physical values in the Force field. The angle values are going higher and higher over generations. Phosphorous is pyramidal in most cases but the FF angle parameter goes towards 180 degrees.

*a40: "[:1]~[#15:2]~[:3]"*	Angle in degrees	k in kcal/mol/rad^2

*a40: "[:1]~[#15:2]~[:3]"*	Angle in degrees	k in kcal/mol/rad^2
Smirnoff99 Frosst	109.5	140
OpenFF 1.0.0	122	149.21
OpenFF 1.2.0	147	139.27
OpenFF 1.3.0	147	163.22
OpenFF 2.0.0	157	129.75
In some experimental fits (dexp ones)	angle is hovering around 170 degrees and in some cases crossed 180 degrees resulting in openmm fails	around 130
In MSM(A+B) starting point refit	119	122.72

MSM mean in blue on Gen2 sets, green plus is refit with MSM starting point, red cross is sage value

The range of angle values on training set molecules that match to a40 is 97 to 120 degrees. Here are some molecules from training set and their QM angles

Angle indices, QM angle
(15, 19, 16) 105
(15, 19, 17) 106
(15, 19, 18) 107
(16, 19, 17) 103
(16, 19, 18) 117
(17, 19, 18) 118

Angle indices, QM angle
(0, 4, 2) 118
(0, 4, 3) 101
(0, 4, 10) 103
(2, 4, 3) 114
(2, 4, 10) 115
(3, 4, 10) 103

Angle indices, QM angle
(12, 17, 14) 104
(12, 17, 15) 120
(12, 17, 16) 112
(14, 17, 15) 102
(14, 17, 16) 109
(15, 17, 16) 109