Introduction
************

As per 2020 UFF force fields and simple classical force fields are
unified in the ForceField engine. This replaces the old UFF engine and
the NewMM functionality from ADF.


Available force fields
======================

The default force field is UFF, it requires no configuration. UFF
(Universal Force Field) is a full periodic table force field that can
be used to calculate single point energies, do geometry optimizations,
calculate frequencies, etc. It is also the default pre-optimizer in
the Graphical User interface (GUI) to clean up manually drawn
structures. For details on the inner workings of the force field, see
the 1992 paper by Rappe et al. [1]. By default UFF uses non-zero
charges for water molecules only.

Other available force fields managed by this engine include Amber95,
Tripos 5.2, GAFF and APPLE&P [9].


ForceField-GUI
==============

Note that the graphical user interface UFF-GUI enables all users to
set up force field calculations with a few mouse clicks. Most force
fields require atom typing, and this needs to be done by hand. When
using UFF, also for pre-optimizing, it is important to remember that
any force field method relies on a set of parameters. This means that
even though UFF supports all elements up to Z=103, it might not
generate the desired structures for uncommon oxidation states in
metallic structures. If this is the case, you could add new parameters
to UFF or attach dummy hydrogen atoms to the metal atom.


What's new in ForceField 2024
=============================

* APPLE&P parameters updated to v 1.13. See also the APPLE&P tutorial
  on ionic liquids.


What's new in ForceField 2023
=============================

* New APPLE&P polarizable force field [9] parameters. See also the
  APPLE&P tutorial on ionic liquids.


What's new in ForceField 2022
=============================

* New APPLE&P polarizable force field [9], designed for polymers,
  electrolytes, ionic liquids and energetic materials. See also the
  APPLE&P tutorial on ionic liquids.

* The GFN-FF force field by Spicher and Grimme. **Note:** GFNFF is
  technically not part of the ForceField engine. To use GFNFF, you
  should use the **GFNFF engine** (and not the ForceField engine). See
  the GFNFF engine manual for more info.


What's new in ForceField 2021
=============================

* The engine has been parallelized with MPI using force decomposition.

* The particle mesh Ewald method (implemented through the helPME
  library) is now used for electrostatic interactions in 3D systems.

* The default non-bonded cutoff has been reduced to 15 Angstrom, a
  value more typical in the force-field community.

* The engine has been optimized to be on par with the industry
  standards. The total speed-up compared to version 2020 can be up to
  factor 500.


What's new in ForceField 2020
=============================


New features
------------

* Periodic support (chains,slabs, and crystals) for all force field
  types

* Import force field parameters from amber ".dat" files

**shell script without atom typing:**

   #!/bin/sh

   # This is a shell script for the ForceField engine

   # You should use '$AMSBIN/ams' instead.

   $AMSBIN/ams <<eor
      # Input options for the AMS driver:

      Task GeometryOptimization

      System
         Atoms
            H 0.0 0.0 0.0
            H 0.9 0.0 0.0
         End
      End

      # The input options for the ForceField, which are described in this manual,
      # should be specified in the 'Engine ForceField' block:

      Engine ForceField
         # the default one is UFF, requiring no options
      EndEngine
   eor


Force field setup for AMS applications
======================================

When you do a geometry optimization of perform a MD calculation the
details of the force field are determined only once and kept constant
during the run. This applies to bond orders, partial charges, etc. In
case of a polarizable force field (not currently supported) the
charges are of course allowed to change.


Classical force fields
======================

Most force fields require atom typing, as well as atomic charge
specifications, see Examples.
