Summary of Functionality

Currently, the QM/MM implementation within ADF is based on a modified version [1] of the 'IMOMM' scheme of Maseras and Morokuma [2] (called the IMOMM/ADF scheme); alternatively, the recently developed AddRemove scheme [3] 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 the implementation:

• Morokuma's IMOMM [2] QM/MM coupling scheme (IMOMM/ADF).
• AddRemove [3] coupling scheme.
• AMBER95 [4] and SYBYL (Tripos) [5] 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, mixed...).
• Any number of covalent bonds can cross the QM-MM boundary.
• 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 atom.
• Not compatible with some ADF extensions such as NMR, ESR, COSMO, IRC and excited states.
• The AddRemove model is available only when using Cartesian coordinates.
• The conjugate gradient optimizer is available only for Cartesian coordinates.