Normally, heats of formation are defined relative to the elements in their standard state. But sometimes more subtle energies are involved, for example, the heat of formation of a hydrogen bond. The easiest way to allow these small energies to be properly represented is to make them functions. Thus the heat of formation of the hydrogen bond in water dimer can most easily be expressed as the heat of formation of the water dimer relative to two isolated water molecules. A highly accurate set of calculations has been done on water and its dimer, and the results can be used as reference data for the energy of the water-water interaction. The "fact" or reference datum here is the difference in energy of the most stable water dimer and the isolated water molecules. Therefore the geometries are not allowed to relax. For the water molecule, the HoF reference is HR=REF, a unique reference that indicates that this is not to be used as a reference function in parameter optimization, instead it is to be used only in building a rule. Here the rule is the definition of the hydrogen bond strength.:
Water molecule, file WATER_REF.mop
SYMMETRY NOLOG Water H=0 HR=REF H 0.00000000 0 0.0000000 0 0.0000000 0 0.0000 O 0.95890000 0 0.0000000 0 0.0000000 0 1 0 0 0.0000 H 0.95890000 0 104.1600000 0 0.0000000 0 2 1 0 0.0000 2 1 3Water dimer is also defined as a frozen geometry. Its heat of formation is defined as -5 kcal per mole plus two times the calculated heat of formation for the WATER_REF system. Now a very large HWT can be used. This increases the weight of the hydrogen bond, without affecting the weights of water or its dimer.
Water dimer Structure 0 (Non-planar open Cs) H=-5.0+WATER_REF+WATER_REF HR=TLHVSQ02 HWT=20 O -0.06679487 0 -1.4898726 0 0.0000000 0 0.0000 O 0.06229548 0 1.4161833 0 0.0000000 0 0.0000 H 0.82318215 0 -1.8446149 0 0.0000000 0 0.0000 H 0.05552209 0 -0.5323432 0 0.0000000 0 0.0000 H -0.40364784 0 1.7732303 0 -0.7592422 0 0.0000 H -0.40364784 0 1.7732303 0 0.7592422 0 0.0000
The relative energies of different configurations can be defined. In the article on water dimers, the energies of various dimers relative to the most stable dimer were reported. These, too, can be used as reference data. The reference file "Water dimer Structure 0 (Non-planar open Cs)" cannot be used, because its heat of formation is defined as a function, instead a new data set is created, "water dimer structure 1 (non-planar open cs).mop":
Water dimer Structure 1 (Non-planar open Cs) H=-120.62 HR=TLHVSQ02 HWT=1 O -0.06679487 0 -1.4898726 0 0.0000000 0 0.0000 O 0.06229548 0 1.4161833 0 0.0000000 0 0.0000 H 0.82318215 0 -1.8446149 0 0.0000000 0 0.0000 H 0.05552209 0 -0.5323432 0 0.0000000 0 0.0000 H -0.40364784 0 1.7732303 0 -0.7592422 0 0.0000 H -0.40364784 0 1.7732303 0 0.7592422 0 0.0000Now the energy difference of one water dimer compared to the most stable water dimer can be defined. Note that since the reference heat of formation is relative to that of a file that contains spaces, "water dimer structure 1 (non-planar open cs).mop" quotation marks are used, and also the relative weight is much larger than normal:
Water dimer Structure 2 (Open Ci) HR=TLHVSQ02 H=0.52+"WATER DIMER STRUCTURE 1 (NON-PLANAR OPEN CS)" HWT=15 O -1.50620978 0 -0.0525869 0 -0.0178046 0 0.0000 O 1.41805430 0 0.0041461 0 -0.0371072 0 0.0000 H -1.85454975 0 0.7608019 0 0.3492898 0 0.0000 H -0.55251212 0 0.0873307 0 -0.0516538 0 0.0000 H 1.72197213 0 -0.7569536 0 0.4618104 0 0.0000 H 2.08418079 0 0.6776128 0 0.1120431 0 0.0000