Hi everyone,
I am running a QM/MM/MD simulation with B3LYP/6-311G level of theory. My QM region is a single dicyanamide (DCA; -1 charge) and the rest of my system (MM region) is composed of various ionic liquid species. Could someone please help me understand why my QUICK Mulliken charges are zero? Below are the details.
I have confirmed that my system is well-equilibrated (w.r.t. temperature, pressure, and density) and have computed diffusion coefficients for the various species in my solution and these agree with previous published values. Thus, I am confident that my system is properly set up.
However, I am struggling to get the QM/MM simulation to print the Mulliken charges of the DCA ion when using QUICK with B3LYP/6-311G. Currently, the .out file reports that the Mulliken charge on each atom in the DCA ion is 0.00. It is unclear where things are going wrong, since the simulation appears to be well-behaved when viewed with VMD. The relevant part of the input with corresponding output charges are here:
Input:
&qmmm
qmmask = ':1',
qmcharge = -1,
qm_theory = 'quick',
qmcut = 10.0,
qmshake = 0,
qm_ewald = 0,
printcharges = 1,
/
&quick
method = 'B3LYP',
basis = '6-311G',
/
Example output with QUICK-B3LYP/6-311G:
Atomic Charges for Step 30 :
Atom Element Mulliken Charge
1 N 0.000
2 C 0.000
3 N 0.000
4 C 0.000
5 N 0.000
Total Mulliken Charge = 0.000
As a test, I switched my electronic structure method to the semi-empirical PM3 hamiltonian that comes with Amber24. Interestingly, this prints charges that are nonzero. Here is the input with a sample of the output:
Input
&qmmm
qmmask = ':1',
qmcharge = -1,
qm_theory = 'PM3',
qmcut = 10.0,
qmshake = 0,
qm_ewald = 0,
printcharges = 1,
/
Example output with PM3
Atomic Charges for Step 740 :
Atom Element Mulliken Charge
1 N -0.191
2 C -0.088
3 N -0.240
4 C 0.015
5 N -0.496
Total Mulliken Charge = -1.000
Could someone please help me understand why my QUICK Mulliken charges are zero? Is there an input that I may be missing?
Thank you all for your time in advance!
Best regards,
Anthony Dominic
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Received on Tue Apr 08 2025 - 08:00:02 PDT
