This keyword is used for Molecular Mechanics (MM) or Quantum Mechanics/Molecular Mechanics (QM/MM) calculations, performed inside the deMon2k program.
FF=$<$String$>$ Force field name as given in the FFDS file.
In MM calculations the potential energy surface of a system is calculated by an empirical force field. The necessary force field parameters are provided in the FFDS file. The FFDS file format follows TINKER [125]. This file can be changed and modified by the user in order to provide alternative force field parameters to deMon2k. In all cases, the first processed entry in this file is the force field name that must be equal to the name given by the FORCEFIELD keyword in the deMon2k input. By default, the FFDS file contains parameters for the OPLS-AA force field published by Jorgensen and co-workers [45,126,127]. Alternatively, a FFDS file for the AMBER force field is also delivered with deMon2k. This file gathers parameters from the different AMBERxx force fields (xx=94, 96, 98, 99, 99sb) [70,128,129,130].

As already described in the GEOMETRY keyword (Section 4.1.1) MM atoms are specified by their atom type number which is given as a negative (-) integer number after the corresponding atomic coordinates or Z-matrix connectivities in the GEOMETRY keyword body. The following shows a single-point energy MM deMon2k input for a water molecule. The negative integer numbers, -76 and -77, after the atomic coordinates specify the atom types for the MM oxygen and hydrogen atoms, respectively. According to the default FFDS file of deMon2k this input defines an SPC water molecule.

 O    -1.628323     0.600651    -1.413703    -76
 H    -1.885969     0.153697    -0.557057    -77
 H    -2.021673     1.495719    -1.360149    -77

Note that the atom connectivities are not specified and are, therefore, automatically generated. Alternatively, connectivities can be provided by the user as described in the GEOMETRY keyword (see 4.1.1).

Figure 5: Definition of QM (red) and MM (blue) regions in a QM/MM input.


The QM/MM approach combines the strengths of QM accuracy and MM speed, allowing the study of chemical processes in solution, proteins and other large systems. For these calculations within deMon2k the FORCEFIELD and QM/MM keywords must be specified and a QM/MM geometry input must be provided. The following shows a QM/MM input for two methanol molecules (Figure 5), the first one is QM (red border in Figure 5) and the second one is MM (blue border in Figure 5). Again the MM connectivity is generated automatically.

 O   96
 H   97
 C   99
 HC  98
 O     1.234808    0.310286   -1.235364
 H     0.282891    0.511411   -1.454987
 C     1.839149   -0.324975   -2.392312
 HC    2.892167   -0.547951   -2.127264
 HC    1.326470   -1.277517   -2.664724
 HC    1.833392    0.354508   -3.275284
 O7   -1.571368    0.671857   -1.491851   -96
 H8   -1.869011    0.181864   -0.675330   -97
 C9   -2.487673    0.359200   -2.572448   -99
 H10  -2.204433    0.996007   -3.434393   -98
 H11  -2.423630   -0.711536   -2.879602   -98
 H12  -3.540363    0.591861   -2.291386   -98

The QM/MM keyword body is mandatory for a QM/MM calculation in deMon2k and will be explained in detail in Section 4.2.4.