I want to set up a calculation to optimize water interaction with a H-bond donor/acceptor as described in the ffTK paper (Figure 2). The only free parameters in this type calculation are:
- The distance between the donor/acceptor and water oxygen
- The rotation of the water molecule about the line connecting the donor/acceptor and water H or O.
Below is one of my attempts at setting this calculation up,
$molecule
0 1
7 -0.401486 -0.000100 0.480833
1 -0.252815 0.000299 1.492676
6 0.325308 1.147493 -0.079578
1 0.101163 2.058770 0.484604
1 -0.030824 1.296429 -1.104572
6 1.824777 0.773671 -0.082546
1 2.316036 1.161200 0.815739
1 2.350981 1.194025 -0.944243
6 1.824608 -0.773720 -0.082574
1 2.315572 -1.161590 0.815149
1 2.350881 -1.194077 -0.945300
6 0.324652 -1.147730 -0.079157
1 0.100616 -2.058030 0.484596
1 -0.031860 -1.296593 -1.104764
1 -2.339592 0.000077 -0.012871
8 -3.267170 0.000162 -0.249158
1 -3.728470 0.000609 0.589552
$end
$rem
JOBTYPE OPT
METHOD HF
BASIS 6-31G*
SCF_CONVERGENCE 8
GEOM_OPT_MAX_CYCLES 100
GEOM_OPT_DRIVER OPTIMIZE
NO_REORIENT TRUE
SYM_IGNORE TRUE
$end
$opt
FIXED
1:14 XYZ
ENDFIXED
CONSTRAINT
stre 15 16 0.9572
stre 16 17 0.9572
bend 15 16 17 104.52
ENDCONSTRAINT
$end
where I have fixed the molecule and constrained the water parameters.
But this takes almost 95 steps to optimize, and I am wondering if there is a more elegant way to set up this type of calculation, maybe by considering the molecule and the water as separate fragments and optimizing only the distance and the rotation (about a bond) between the fragments.