From time to time, users send recurring questions to our Help Desk. Here are a few examples of those, along with their responses.

Question: I'm experimenting with Gaussian's PBC capabilities and am surprised by how much memory and disk space is required for a simple HF/6-31G** calculation. Decreasing the number of k-points does not seem to help much. Is there anything I can do to reduce the memory and disk requirements?

Answer:
Reducing the number of k-points will not significantly alter the required resource requirements and may lead to a poor description of the electronic structure of the system. For efficiency considerations, pure DFT functionals are recommended for all preliminary PBC work. Unlike in the case for calculations on molecules, Hartree-Fock and hybrid DFT calculations on periodic systems are much more costly in terms of system resources and CPU time than are DFT calculations using pure functionals and density fitting. For example, a PBC calculation using BLYP/6-31G(d,p)/Auto will require significantly fewer resources than a calculation using HF/6-31G(d,p).

Question: Can Gaussian perform density of states or band structure calculations?

Answer:
At the moment there is no automated DOS or band structure output, just the basic eigenvalues at each k-point output. However,  G03 revision B.04 and earlier will print out the highest four occupied eigenvalues and lowest four unoccupied eigenvalues at each k- point as part of the debug output if you include the keyword IOp(5/33=3) in the job's route section.

Starting with revision B.05, even more information is available:

• When using #P, the k-point and values of the highest occupied and lowest virtual energies are printed at the end of the SCF, along with the largest and smallest direct band gap and the indirect band gap.

• If you include IOp(5/103=N) in the route section , then the energies of the N highest occupieds and N lowest virtuals at each k-point will be printed. If N≤5, these items appear on one line per k-point; if N>5, then the occupied and virtual energies are printed on separate lines.

Question: I have a G03 source license. Do I have to purchase a separate unlimited binary license to receive precompiled binary versions?

Answer:
No. A source license entitles you to request any of the corresponding binary versions you need, at the cost of the media fee. Note that you may request only those versions included with your source code; for example, an academic source code user may request an IA32/Linux binary but not a NEC SX binary since the latter is available only with a commercial source code license.