Hello all
Regarding the issue of heat and temperature, calculations are not
trivial, but nor need they be so daunting.
For some materials, such as molecular crystals, or silicates
(minerals, including clays and zeolites), there are very good
empirical potentials, for which you can use gulp provided that
the phases do not suddenly become dynamically disordered. To
apply pressure is easy - the program minimises the enthalpy,
which is the energy of the interatomic interactions and the PV
term. Gulp can also minimise the free energy, which is enthalpy
+ the phonon free energy, and thereby give you thermal expansion.
The results can be surprisingly good (and to save time some
examples are floating around my web site).
For other cases, the empirical potentials may need to be derived,
and this may or may not be trivial/easy.
Large computers are not needed for this approach.
If you have dynamic disorder, as in the high-T phase of cristobalite,
gulp will not help at all. If you can use empirical potentials,
the technique is molecular dynamics, and there are several codes
around. Larger computers are needed for this, but they need not
be supercomputers.
If empirical potentials are really not good enough, then you come
to the quantum mechanical calculations that Larry Finger refered to.
These cannot be tackled by the novice without help, and do require
a lot of computing resources. Well, for simple systems a small
workstation is okay. They actually are not always more
accurate that when using empirical potentials, but they probably
start to win when interatomic distances get smaller than the range
of interatomic distances using in the development of empirical
models. You can do molecular dynamics with quantum mechanics,
but only on very small samples, and I reckon that you then feel
the effects of finite sizes.
I think that modelling goes hand-in-hand with crystallography,
and although this is a mailing list devoted to one aspect of
crystallography, it is nice to see modelling aired on this
mailing list!
Martin Dove
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Martin Dove
Mineral Physics group, Department of Earth Sciences,
University of Cambridge, Downing Street, Cambridge CB2 3EQ
Tel. 01223-333482 or 01223-740075. Sec: 01223-333459. Fax: 01223-333450
email: [EMAIL PROTECTED] WWW: http://www.esc.cam.ac.uk/astaff/dove
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