ADVANCED USERS DMACRYS & NEIGHCRYS manual Manual ...

analytical second derivatives being generated in the relaxation step, because this is faster and usually almost
as accurate.
Error messages
Error - cannot locate a lower energy along a negative eigenvalue
This indicates that even the smallest displacement selected by DMACRYS was too large, and none of the
points investigated were lower in energy than the original saddle point structure.
THE FORCE CONSTANT MATRIX IS AS FOLLOWS
THE GRADIENT VECTOR IS AS FOLLOWS
The force constant matrix and gradient vector may be printed by setting PRIN PLUT 01000. If just the
gradient vector is required, set PRIN PLUT 03000 (first and last cycle only) or PRIN PLUT 04000 (Each
cycle). There are 6*MOLS + 6 elements. The order of the elements is
x, y and z derivatives of each molecule in the sorted order. This is followed by the torsions for each
molecule.
6 components of the bulk strain derivatives.
Note that there are mixed second derivatives between all three types. The elements of the matrix are not
scaled to eV Angstrom units. To convert to eV Angstrom units, multiply
W or G Position Multiplication Resulting Units
From
To
factor
Gradient 1 6*MOLS
14.3994584 (eV Å) -1
2
RLAT
6*MOLS
ENDS
14.3994584 (eV Å) -2
3
RLAT
Matrix 1,1 6*MOLS,6*MOLS 14.3994584 (eV Å) -2
3
RLAT
1,6*MOLS
6*MOLS,END 14.3994584 (eV Å) -3
4
RLAT
6*MOLS,6*MOLS END,END
14.3994584
5
RLAT
(eV Å) -4
The lattice vectors, basis species types and positions will be written to FORTRAN unit 8. This is by default
for DMACRYS.
3.10 PROPERTIES CALCULATIONS
The elastic stiffness tensor 1 and k=0 phonon frequencies 2 of the perfect crystal at 0 K are estimated within
DMACRYS from the second derivatives of the energy with respect to rigid body motions. The omission of
the NOPR directive means that the full second derivative matrix is calculated numerically from the
analytical gradients. The time taken to carry out the calculations is increased significantly.
The calculation should start from a lattice energy minimum; otherwise the relaxation may change the
conventional Cartesian reference frame, leading to an error in the definition of the elastic constants. The
restarting of the calculation may lead to insignificant small changes. The values that are changed very
slightly by the NOPR directive being omitted are:
• Ewald summed charge-charge energy
• Inter-molecular charge-charge energy
• Total charge-dipole energy
• Total dipole-dipole energy
• Total charge-dipole+dipole-dipole energy
• Higher multipole interaction energy
• Total isotropic repulsion-dispersion
• The Eigenvalues for the representations
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