Hi DS Strain is a geometrical property. If you want a fast calculation, just use the Bilbao Crystallographic Server. http://www.cryst.ehu.es/cryst/strain.html there you may input two sets of lattice parameters and get the strain relating both lattices.
Take a look at Fast et al, PRB 51, 17431 (1995). In equations (1) and (2) are explained the relation between lattice vectors and strain. In Kittel's textbook it is explained too, but I am not sure if in all the editions. In the article of Fast it explains what you can do with Q-E and similar codes. In Q-E you input the lattice parameters or the lattice vectors, and all the other stuff, and get the stress tensor as explained in previous posts. Maybe what you need is to find the strain of a crystal under an specified stress tensor. As far as I know, with Q-E you cand do it only in the case of hydrostatic stress (when the stress tensor is the pressure times the identity matrix), using calculation='vc-relax' and setting the variable press. E.g. &control calculation = 'vc-relax' .......... &CELL cell_dynamics='damp-pr', press = 35.0, (the value of pressure in kbars) cell_dofree='all', For a non-hydrostatic stress, you may need to find the elastic constants (as in Fasts's paper) first and then solve the equations of elasticity. If the stress is big, you may need an iterative process. If you need the strain under a fully specified stress tensor, i.e., the six independent components of the tensor, you may device a minimization algorithm implemented in a shell or python script, that run pw.x to get the energies and stress. An alternative is to find the elastic constants (as in Fasts's paper) first and then solve the equations of elasticity. If the stress is big, you may need an iterative process to find the elastic constants under a stress that is close to your derired stress. In simple cases, such as uniaxial stress and orthorombic lattices, you do cell relaxations with constraints (cell_dofree='xxxxx' ) keeping one lattice vector fixed, and by trial and error you can obtain the lattice that produce the desired component of the stress. -- Eduardo Menendez Departamento de Fisica Facultad de Ciencias Universidad de Chile Phone: (56)(2)9787439 URL: http://fisica.ciencias.uchile.cl/~emenendez -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20090708/4abbc09b/attachment.htm