The mobile block Hessian (MBH) method [282, 283] is useful when calculating vibrational frequencies of a small part of a very large system (molecule or cluster). Calculation of the full spectrum of such a system may be inefficient and is unnecessary if one is interested in one particular part. Besides, it may be difficult to extract normal modes related to the interesting sub-system out of the whole spectrum. Using MBH it is possible to treat parts of the system as rigid blocks. Each block will usually have only six frequencies related to its rigid motions compared to 3*N for when each atom of the block is treated separately.
The calculation of frequencies using mobile blocks is invoked by specifying FREQUENCIES and MBH keywords at the same time in the GEOMETRY input block:
GEOMETRY
FREQUENCIES
MBH blockname1 blockname2 ...
End
The names of blocks must correspond to the ones specified in the b= parameter in the ATOMS input block.
The second derivatives with respect to block motions are calculated by numerical differentiation. Since the number of degrees of freedom is reduced, the number of second derivatives is reduced as well. Therefore the MBH can realize a speed-up in the calculation of the Hessian compared to a full numerical frequency calculation.
MBH for partially optimized structures
