Note: This post will be moved to Force Field / FF releases / Parsley minor release(s) once it is ready to be shared.
Fitting Data and Results
Fitting targets: 2nd generation training sets (link for the details of the training set generation scheme: http://doi.org/10.5281/zenodo.3777278)
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Angle terms with noticeable different k values between RC1 and RC2:
a6([#1:1]-[*;r3:2]~;!@[*:3], k value in SMIRNOFF99Frosst: 100 kcal/mol/radian2)a3([*;r3:1]1~;@[*;r3:2]~;@[*;r3:3]1, k value in SMIRNOFF99Frosst: 200 kcal/mol/radian2)a15([#8X1:1]~[#6X3:2]~[#8:3], k value in SMIRNOFF99Frosst: 126 kcal/mol/radian2)
: Based solely on intuition, RC2 angle k values for the angle terms(doesn’t change much from the k value in 1.1.0 throughout the optimization) seem physical; 400kcal/mol/radian2 (final k value in RC1) for angle seems too large compare to other angle k values. Final gradients for the angle k values are also higher in RC1 (a6: 5.580e+00 , a15 : 4.750e+00) compared to the gradients in RC2 (a6: 1.599e+00 , a15 : 1.391e+00)
: Scatter plots seem slightly better in RC1. Also one thing I noticed is that large initial guess of a15 equilibrium angle has been used in RC2 fitting, which led to larger final equilibrium angle(~135 degree) compared to the final equilibrium angle in RC1(~ 129 degree). (a15 equilibrium angle in v1.1.0 is around 137 degree, which is larger than angles observed in QM optimized geometries.)
( The issue with converging to a final equilibrium value substantially off from values in QM optimized geometries is one of the known issues that we are currently working on. )
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Cf. But it seems like having final equilibrium angle different with QM values is not always the case we want to avoid. Here’s one example. As you can see in the scatter plot below, the fitting converged to a larger a38 equilibrium angle (~ 147 degree) which is larger than QM values(~ 110 degree). And the large equilibrium angle has been found to be beneficial in locating hydroxyl hydrogen in phosphono group far away enough from its neighboring oxygens, preventing unphysical intermolecular H-bonding between the hydrogen and the neighbor oxygens.
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Torsion terms with significant different final k values between RC1 and RC2:
t146,t147: having 6 cosine functionst15([*:1]-[#6X4;r3:2]-@[#6X4;r3:3]-[*:4]),t16([#6X4;r3:1]-[#6X4;r3:2]-[#6X4;r3:3]-[*:4]): in-ring rotations
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Specific improvement in certain functional groups(phosphono group, sulfamate acetate) found in RC1 is also shown in RC2.
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QM optimized geometry of CC(O)([P@@](=O)(O)[O-])[P@](=O)(O)[O-]. ( orange: MM optimized geometry with v1.1.0 force field, green: v1.2.0-RC1 force field, magenta: v1.2.0-RC2 force field)
(3) v1.2.0-RC1 vs. v1.2.0-RC2: Relative energies between conformers at “QM optimized geometries”
Comparison of performances of RC1 and RC2 in reproducing QM relative energies between conformers was carried out. Two different ways to calculate MM relative energies were used. Two different ways to calculate MM relative energies were used. For the left figure, MM relative energies were calculated by taking a difference between MM energy at each point and MM energy at the QM minimum. And for the right figure, MM relative energies were obtained by taking a difference between MM energy at each point and MM energy at QM minimum. Both candidates have smaller MAD and shorter tails than v1.1.0, indicating slight better performances over v1.1.0.
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