HF riper: warning to enlarge simulation cell

[kesha]

Hello,

I am getting started with using the riper module of Turbomole. I ran a few calculations from the Turbotest folder. The calculations worked fine when I ran riper with DFT but when I tried with HF, I see the following warning (pertaining to the HF exchange it seems):
               +-------------------------------------------------------+
              |            !!!!!!  WARNING   !!!!!!                       |
              +--------------------------------------------------------+
              |                                                                     |
              |           Your crystal is very small!                  |
              |       Better enlarge it, by using a larger         |
              |        simulation box or increased number     |
              |                   of k-points.                                 |
              |        Enlarge k-mesh or simulation cell!        |
              |                                                                      |
              +---------------------------------------------------------+
So, firstly, should I be bothered by this warning?
From the manual I understand how I can provide a k-point mesh and providing a large k-point mesh does help but when I am doing a Gamma point calculation, what is the way to enlarge the simulation cell since I don't need to provide a k-point mesh in that case? (For eg. I saw this warning with NaCl.3D.Gamma point calculation and MgO.3D SCF calculation which are given in the Turbotest folder)

[mas]

Hello,

which Turbomole version do you use? HF calculations for periodic systems using gamma point only are only meaningful for very large supercells. Otherwise you have to use k point sampling or your results will be simply wrong.
Please read the following paper for more information: https://pubs.acs.org/doi/10.1021/acs.jctc.8b00122

[kesha]

Thank you for the information Prof. Sierka. That helps. I am using Turbomole version 7.4.