# How many GB of RAM would be needed for VASP to evaluate optical properties of a material?

Even though it is a powerful tool, Quantum ESPRESSO's hardware requirements are very high. For instance, to evaluate optical properties, even for structures with tens of atoms, Quantum ESPRESSO can require more than 100GB of RAM. What about VASP?

• What do you mean by hardware requirements? You can run QE on a laptop and it's been even run on an Android phone. – Kevin J. M. Nov 23 '20 at 1:31
• @KevinJ.M., sure, for very small system. QE is a high RAM dependent. – Camps Nov 23 '20 at 11:31
• Exactly @Camps! To evaluate optical properties, for example, even to structures with tens of atoms, Quantum Espresso can take more than 100Gb of RAM. Therefore running on mobile can not be the case! – Anibal Bezerra Nov 23 '20 at 17:33
• My point is: as it was, the question could not be answered clearly (Nike has now edited it to make it more specific). Also the RAM requirements depend on other parameters in the calculation, such as how much data is stored in on disk (disk_io). And what specific approach is being used to calculate optical properties here? – Kevin J. M. Nov 24 '20 at 0:02

This question is kind of general. I will outline some points related.

• Firstly, it depends on the theoretical levels that you use to calculate the optical properties. In general, there are three levels:

• Independent level;
• RPA (random phase approximation) level;
• GW&BSE level;

The first one is memory friendly in terms of the empty bands contained. However, the last two are very memory-consuming even for a small system.

• Secondly, it depends on how large your system and whether includes the spin-orbit coupling or not.

• Thirdly, you must test the parameters to obtain converged results. All these parameters are closely related to memory consumption. In VASP, the key parameters are:

• KMESH
• ENCUT
• BANDS & ENCUTGW
• NBANDSV
• NBANDSO

For your example with tens of atoms, to obtain converged results I believe 100G memory is not enough in VASP. In summary, you can test it and get some feeling.

• Is it normal that a calculation for TENs atoms require more that 100GB of RAM? – Camps Nov 25 '20 at 11:41