External Fields

In addition to the molecular force field, DL_POLY_4 allows the use of an external force field. Examples of fields available include:

1. Electric field: (elec)

   \[\underline{F_{i}} = \underline{F_{i}} + q_{i} \cdot \underline{E}\]

2. Oscillating shear: (oshr)

   \[\underline{F}_{x} =A \cos(2n \pi \cdot z/L_{z})\]

3. Continuous shear: (shrx)

   \[\underline{v}_{x} = \frac{1}{2} A \frac{|z|}{z}~~~~~:|z|>z_{0}\]

4. Gravitational field: (grav)

   \[\underline{F_{i}} = \underline{F_{i}} + m_{i} \cdot \underline{G}\]

5. Magnetic field: (magn)

   \[\underline{F_{i}} = \underline{F_{i}} + q_{i} \cdot (\underline{v_{i}} \times \underline{H})\]

6. Containing sphere: (sphr)

   \[\underline{F}=A~(R_{0}-r)^{-n}~~~~~: r>R_{\rm cut}\]

7. Repulsive wall: (zbnd)

   \[\underline{F}_{z}=A~(z_{o}-z)~~~~~: f \cdot z > f \cdot z_{o}~~,\]

   where \(f=+/-1\) with default of \(1\).

8. X-Piston: (xpis)

   \[\underline{F}_{x} = \frac{m_{k}}{\sum_{k=i}^{j}m_{k}} P \cdot \underline{\textrm{Area}}(\perp\textrm{X-}direction)~~~~~: \forall~k=i,..,j~~.\]

9. Harmonic restraint zone in z-direction: (zres)

   \[\begin{split}\underline{F}_{z} = \left\{ \begin{array} {l@{\quad:\quad}l} A~(z_{com}-z_{max}) & z_{com} > z_{max} \\ A~(z_{min}-z_{com}) & z_{com} < z_{min}~~, \end{array} \right.\end{split}\]

   where \(z_{com}\) is the chosen molecule centre of mass.

10. Harmonic restraint zone in z-direction \(-\) (push out): (zrs:math:`-`)

    \[\begin{split}\underline{F}_{z} = \left\{ \begin{array} {l@{\quad:\quad}l} A~(z-z_{max}) & z \ge (z_{max}+z_{min})/2 \\ A~(z_{min}-z) & z < (z_{max}+z_{min})/2 \end{array} \right.\end{split}\]

11. Harmonic restraint zone in z-direction + (pull in): (zrs+)

    \[\begin{split}\underline{F}_{z} = \left\{ \begin{array} {l@{\quad:\quad}l} A~(z-z_{max}) & z > z_{max} \\ A~(z_{min}-z) & z < z_{min} \end{array} \right.\end{split}\]

12. Oscillating electric field: (osel)

    \[\underline{F_{i}} = \underline{F_{i}} + q_{i} \cdot \underline{E} \cdot \sin(2 \pi \omega t)~,\]

    where \(t\) is the simulated time.

13. Umbrella sampling (harmonic restraint) ^48,116: (ushr)

    \[U_{AB} = \frac{k}{2} (R_{AB}-R_{0})^{2}~,\]

    is an umbrella sampling harmonic restraint between the centres of masses of two molecules (non-overlapping clusters of particles), \(A\) and \(B\), with a force constant, \(k\), and an equilibrium distance, \(R_{0}\).

It is recommended that the use of an external field should be accompanied by a thermostat (this does not apply to examples 6 and 7, since these are conservative fields). The “Oscillating shear” and “X-piston” fields may only be used with orthorhombic cell geometry (imcon\(=1\),:math:2`) and “Continuous shear” field with slab cell geometry (imcon\(=6\)).

In the case of the “X-piston” field it is strongly advised that the number of piston particles is chosen to be very small in comparison with the rest of the system (\(< 5\%\)) and that the piston contains its own set of whole molecules (i.e. there are no molecules partially mapped on the piston), which do not include any core-shell, CB, PMF or RB units! The field releases the system’s centre of mass to move unconstrained and gain momentum. This makes any temperature control options control the full kinetic energy of the system and thus the only ensemble valid under this conditions and possible within DL_POLY_4 at the present is the micro-canonical (NVE)!

The user is advised to be careful with the parameters’ units! For more insight, do examine Table (%s) and the example at equation (171) in Section The FIELD File.

In DL_POLY_4 external field forces are handled by the routines external_field_apply and .