Selecting the DMA: UseDMA

Synopsis

[-]usedma 1|2|3|4|5|6|7|8|9

Description

The keyword specifies which DMA has to be used in the calculation of molecular properties (moments and interaction energies). Currently, PAMoC can store up to eight different DMAs, but only one of these is used in the calculations. The DMA used by default is the one retrieved from the interface data file (IDF). This can be replaced by any DMA, specified through the keyword “[-]usedma <n>”. It should be emphasized that the substitution will only affect the calculation of those properties that explicitly depend on DMA. The remaining properties still rely on the electronic density provided by the IDF.

It is up to the user to provide PAMoC with other DMAs in addition to that supplied by the IDF. This is accomplished through a number of appropriate keywords (see below).

If the IDF is a wavefunction file (.wfn), PAMoC generates two DMAs. The first is a generalization of the Mulliken population analysis. The second is obtained using the Stone nearest-site allocation algorithm. The Mulliken DMA (n = 1) is assumed as the default in subsequent calculations. The Stone DMA can be selected by setting “[-]usedma 2”.

As stated above, the selected DMA is used to calculate properties that explicitly depend on DMA, i.e. molecular electrostatic moments and interaction energies. However PAMoC evaluates molecular electrostatic moments using all the available DMAs. This is done to test the accuracy of the partitioning schemes of the electron density that yielded the different DMAs. Indeed, while different partitioning schemes may lead to quite different DMAs of the electron density of a molecule, the calculation of molecular moments from each of these DMAs must lead to the same result. This is like a jigsaw puzzle which is created by cutting a picture into small pieces with a jigsaw. From the same picture you can get many puzzles which differ from each other for the number and shape of interlocking and tesselating pieces, but all these puzzles will show the same picture when the assembling of all pieces is complete. This fact is illustrated in the Figure.

20-pieces puzzle — **Figure** − The whole picture is left unchanged by different partitioning schemes.

63-pieces puzzle — **Figure** − The whole picture is left unchanged by different partitioning schemes.

In conclusion, the choice of different DMAs can be useful only in the calculation of intermolecular electrostatic energies of interaction, crystal cohesion energies, and electrostatic moments of molecular fragments.