Summary of Functionality
Currently, the QM/MM implementation within ADF is based on a
modified version  of the 'IMOMM' scheme of Maseras and Morokuma 
(called the IMOMM/ADF scheme); alternatively, the
recently developed AddRemove scheme  is available. The molecular mechanics code has been
designed to be as flexible as possible, allowing for many levels of
customization. As a result of this
flexibility, operation of the program requires the user to have some experience
with molecular mechanics methods. At
the same time, ADF remains the main driver to control the simulation of the whole
QM/MM system, since one of the objectives of the implementation has been to
treat the MM subsystem as a perturbation to the QM system.
We summarize the current functionality and limitations of
- Morokuma's IMOMM  QM/MM coupling scheme (IMOMM/ADF).
- AddRemove  coupling scheme.
- AMBER95  and SYBYL (Tripos)  force fields provided.
- Free form and modifiable force field parameter file.
- Ability to mix and match force field functions.
- Geometry optimization, linear transit and transition
state optimizations (In these optimizations,
the MM region is fully optimized between each SCF cycle or in
other words between each QM geometry step).
- All ADF input 'styles' available (Cartesian, Z-matrix,
- Any number of covalent bonds can cross the QM-MM
- Global optimization of the MM subsystem available
(simulated annealing and grid search algorithms).
Some notable limitations are:
- Symmetry constraints cannot be applied, unless the MM
subsystem is frozen.
- Geometry constraints involving MM atoms are limited.
- Not more than one QM atom can be bonded to a single MM
- Not compatible with some ADF extensions such as NMR,
ESR, COSMO, IRC and excited states.
- The AddRemove model is available only when using
- The conjugate gradient optimizer is available only for