3 Mean Square Displacement
4 ------------------------
6 | :ref:`gmx msd <gmx msd>`
7 | To determine the self diffusion
8 coefficient :math:`D_A` of
9 particles of type :math:`A`, one can use the Einstein
10 relation :ref:`108 <refAllen87>`:
12 .. math:: \lim_{t \rightarrow \infty} \langle
13 \|{\bf r}_i(t) - {\bf r}_i(0)\|^2 \rangle_{i \in A} ~=~ 6 D_A t
16 | This *mean square displacement* and :math:`D_A` are calculated by the
17 program :ref:`gmx msd <gmx msd>`. Normally
18 an index file containing atom numbers is used and the MSD is averaged
19 over these atoms. For molecules consisting of more than one atom,
20 :math:`{\bf r}_i` can be taken as the center of mass positions of the
21 molecules. In that case, you should use an index file with molecule
22 numbers. The results will be nearly identical to averaging over atoms,
23 however. The :ref:`gmx msd <gmx msd>` program can also be used for
24 calculating diffusion in one or two dimensions. This is useful for
25 studying lateral diffusion on interfaces.
27 An example of the mean square displacement of SPC water is given in
28 :numref:`Fig. %s <fig-msdwater>`.
32 .. figure:: plots/msdwater.*
35 Mean Square Displacement of SPC-water.