LEPSILON= .TRUE. | .FALSE.
Determines the static ion-clamped dielectric matrix using density functional perturbation theory. The dielectric matrix is calculated with and without local field effects. Usually local field effects are determined on the Hartree level, i.e. including changes of the Hartree potential. To include microscopic changes of the exchange correlation potential the tag LRPA=.FALSE. must be set (see Sec. 6.72.5). The method is explained in detail in Ref. , and follows closely the original work of Baroni and Resta. A summation over empty conduction band states is not required, as opposed to the method selected by setting LOPTICS=.TRUE. (see Sec. 6.72.1). Instead, the usual expressions in perturbation theory
N E dE d eps ncg rms rms(c) RMM: 1 -0.14800E+01 -0.85101E-01 -0.72835E+00 220 0.907E+00 0.146E+00 RMM: 2 -0.14248E+01 0.55195E-01 -0.27994E-01 221 0.449E+00 0.719E-01 RMM: 3 -0.13949E+01 0.29864E-01 -0.10673E-01 240 0.322E+00 0.131E-01 RMM: 4 -0.13949E+01 0.13883E-04 -0.31511E-03 242 0.600E-01 0.336E-02 RMM: 5 -0.13949E+01 0.28357E-04 -0.25757E-04 228 0.177E-01 0.126E-02It is important to note that exact values for the dielectric matrix are obtained even if only valence band states are calculated. Hence this method does not require to increase the NBANDS parameter. The final values for the static dielectric matrix can be found in the OUTCAR file after the lines
MACROSCOPIC STATIC DIELECTRIC TENSOR (excluding local field effects)and
MACROSCOPIC STATIC DIELECTRIC TENSOR (including local field effects in DFT)The values found after MACROSCOPIC STATIC DIELECTRIC TENSOR (excluding local field effects) should match exactly to the zero frequency values determined by the method selected using LOPTICS=.TRUE. (see Sec. 6.72.1). This offers a convenient way to determine how many empty bands are required for LOPTICS=.TRUE.. Simply execute VASP using LEPSILON=.TRUE. in order to determine the exact values for the dielectric constants. Next, switch to LOPTICS=.TRUE. and increase the number of conduction bands until the same values are obtained as using density functional perturbation theory.
Note that the routine also parses and uses the value supplied in the LNABLA tag (see Sec. 6.72.3). Furthermore, the routine calculates the Born effective charge tensor (dynamical charges) and electronic contribution to the piezoelectric tensor, and prints them after
BORN EFFECTIVE CHARGES (in e, cummulative output)and
PIEZOELECTRIC TENSOR for field in x, y, z (C/m^2)if LRPA=.FALSE. is set (the calculated tensors are not sensible in the random phase approximation LRPA=.TRUE.).
Pros compared to LOPTICS=.TRUE. (see Sec. 6.72.1):