Keyword QM/MM

This keyword defines the mechanical interaction between the QM and the MM region in a QM/MM calculation. To this end atom types of the force field are assigned to the QM atoms.
Options:

	CHARMM		The CHARMM-deMon2k interface is activated.
	NOPOLES		Disables the asymptotic expansion of QM/MM embedding integrals.

Description:
The mechanical interaction between the QM and MM regions is expressed by a Lennard-Jones potential, Eq. (4.13) in Section 1.4. Therefore, Lennard-Jones parameters must be assigned to the QM atoms. They are taken from the force field. To do so, atom types are assigned to the QM atoms with the QM/MM keyword. These atom types serve only for the assignment of van der Waals radii and potential depths. They are defined in the keyword body of the QM/MM keyword as shown in the following input example. Note that they are assigned by positive integer numbers.

 TITLE QM WATER AND MM WATER
 #
 FORCEFIELD FF=OPLS-AA
 QM/MM
 O   76
 H   77
 #
 GEOMETRY CARTESIAN ANGSTROM
 O     1.187632     0.332759    -1.195914
 H     0.217713     0.505507    -1.369657
 H     1.503324    -0.129952    -1.999029
 O    -1.628323     0.600651    -1.413703    -76
 H    -1.885969     0.153697    -0.557057    -77
 H    -2.021673     1.495719    -1.360149    -77

The QM atoms can be addressed in the QM/MM keyword body by a specific atomic symbol or by atom groups using the element symbol.

With the CHARMM option of the QM/MM keyword the deMon2k CHARMM interface is activated. In such calculations CHARMM drives the QM/MM calculations and deMon2k calculates only energies and gradients for a given, electrostatically embedded, QM system. The following deMon2k input shows again a QM/MM water dimer, now for a CHARMM-deMon2k calculation.

 TITLE CHARMM QM WATER AND MM WATER
 #
 VXCTYPE PBE
 QM/MM CHARMM
 MATINV ANALYTICAL
 #
 EMBED READ
       0.88590000      0.36292000      0.37595000     -0.83400000
       1.25000000      0.75598000      1.17374000      0.41700000
       0.28602000      1.03645000      0.04525000      0.41700000
 GEOMETRY
 O            2.3563900000       -1.4195200000       -1.1274700000
 H            1.8724500000       -0.7537800000       -0.6025600000
 H            1.7965500000       -2.1894500000       -1.0546000000

Here the MM water is given in the keyword body of the EMBED keyword by coordinates and point charges. The QM water geometry is given as usual in the keyword body of the GEOMETRY keyword.

Processing this input with deMon2k will create, besides the usual output files, a QM/MM interface file, deMon.qmm, that has the following format:

 TOTAL ENERGY                =        -76.3904859540375

 QMFORCES
    -0.115663E-01    -0.150402E-01     0.179275E-02
     0.716778E-02    -0.110224E-01    -0.794221E-02
     0.111375E-01     0.189759E-01    -0.250127E-03

 EMBEDFORCES
    -0.904640E-02     0.126830E-01     0.978137E-02
     0.650241E-04    -0.258697E-02    -0.295494E-02
     0.203853E-02    -0.304493E-02    -0.311790E-03

 FINAL INPUT ORIENTATION
  NO.  ATOM        X             Y             Z        Z-ATOM   MASS
    1  O        2.356390     -1.419520     -1.127470       8    15.999
    2  H        1.872450     -0.753780     -0.602560       1     1.008
    3  H        1.796550     -2.189450     -1.054600       1     1.008

This file contains the energy of the QM system, $E^{QM}$ in Eq. (4.9) of Section 1.4, and the corresponding forces on the QM (QMFORCES) and MM (EMBEDFORCES) atoms. At the end of the file the final input orientation of the QM system is also given.

The option NOPOLES of the QM/MM keyword disables the asymptotic expansion of the electrostatic embedding integrals, Eq. (4.10), in Section 1.4. For large MM regions this can slow down the calculation and, therefore, should only be used for testing and validation.