G=G'=0 term in the long-range contribution (LRC) to exchange-correlation kernel in Sternheimer approach
The missing G=G'=0 term is added to the LRC kernel in Sternheimer calculations of optical and magneto-optical polarizability. The polarizability calculated with the kernel without G=G'=0 term is just multiplied by the coefficient depending on this polarizability following equations on page 2 of R. Stubner et al. Phys. Rev. B 70, 245119 (2004).
When the LRC kernel is used, the dielectric constant without G=G'=0 term is still included in the output. This is for the case when a large number of k-points through shifted k-point grids is required. Then it can be better first to compute the total polarizability without G=G'=0 term from all the grids and apply it for calculation of the G=G'=0 term at the post-processing stage.
The description of the XCKernelLRCAlpha variable is updated correspondingly.
The XCKernelLRCAlpha = 0.2 is included in the test on magneto-optics of silicon. The reference data in the test are updated to include the output with and without G=G'=0 term.