mirror of https://gitlab.com/QEF/q-e.git
76 lines
3.6 KiB
Plaintext
76 lines
3.6 KiB
Plaintext
The code phcg.x contained in the Gamma/ subdirectory is a specialized
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phonon code, calculating phonon modes only at Gamma (q=0), and using
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only Gamma (k=0) for the sum over the Brillouin Zone. It performs direct
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minimization of the energy functional expanded at second order in the
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atomic displacements. The code was written having molecular applications
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in mind : Raman and IR spectra calculations in C60 (JCP100, p.8537, 1994)
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and later in models of the GFP chromophore (Chem.Phys. 287/1-2, p.33, 2003).
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The (non resonant) Raman cross sections are calculated by finite differences
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of the dielectric tensor with respect to small atomic displacements. This
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method is simple but slow. The Raman calculation using such as second-order
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response, as implemented in code ph.x, should be much faster. The calculation
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of Raman cross sections in phcg.x was retained for testing purposes, or for
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some special cases (high-symmetry molecules for instance).
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Note that:
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- the code reads a file produced by pw.x with Gamma point only and
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Gamma-point special tricks (half of the plane waves and so on)
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- Ultrasoft PP's are not implemented
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- the symmetry is used in a different way wrt all other PWscf codes:
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only inequivalent atoms are displaced. This has an historical reason:
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C60 has just 3 inequivalent atoms when put into in a cubic cell with
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standard orientation (the isolated molecule has 1 inequivalent atom)
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- the code saves partial and final results in files "restart_e" and
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"restartph". Specify "recover=.true." in the input data in order to
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restart from saved data.
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Input:
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title (a string of characters)
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&inputph [variable1=value1, variable2=value2,...] /
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displacement patterns (optional, see below)
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Input variables as in the phonon code:
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outdir
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prefix
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fildyn
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epsil
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trans
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recover
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tr2_ph Note that Conjugate-Gradient algorithm stops when
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|| A\delta\psi - B || < tr2_ph, where A and B define
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the DFT functional expanded at second order:
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E^{(2)} = (1/2) (\delta\psi A \delta\psi) - B\delta\psi
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Variables specific to this code:
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asr use Acoustic Sum Rule to reduce by 3 the number of phonon
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calculations to be performed (used only if trans=.true.)
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nmodes use the specified displacement patterns, provided after the
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namelist as nmodes vectors, each with 3*nat components
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(nat=number of atoms); nmodes < 3*nat is allowed (I think).
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If not set, uses all 3*nat displacement patterns (one atom
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at the time, along x, or y, or z)
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Raman-specific variables for this code:
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raman perform a calculation of Raman cross sections by finite
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differences
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deltatau finite difference (a.u.): typically a small but not too
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small amount, something like 0.01 a.u, or so
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nderiv type of finite derivative formula: nderiv=2 is a simple
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two-point formula (two calculations per displacement)
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[ f (-deltatau) + f(+deltatau) - 2 f(0) ] / 2/ deltatau^2
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nderiv=4 is a four-point formula (4 calculations/displacement)
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first, last
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calculate raman intensities only for phonon modes from the
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"first" to the "last" (in increasing order of energy, as
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produced by the code after diagonalization of the dynamical
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matrix), using these phonon modes as displacement patterns.
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Requires nderiv*(last-first+1) calculations of the dielectric
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tensor. If not set, first calculate all derivatives of the
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dielectric tensor wrt atomic displacements (nderiv*3*nat
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calculations); then, Raman intensities for all phonon modes.
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