|FF=String||Force field name as given in the FFDS file.|
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.
TITLE SPC WATER # FORCEFIELD FF=OPLS-AA # GEOMETRY CARTESIAN ANGSTROM 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).
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.
TITLE QM/MM CH3OH Dimer # FORCEFIELD FF=OPLS-AA QM/MM O 96 H 97 C 99 HC 98 # GEOMETRY CARTESIAN ANGSTROM 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.