[Pw_forum] How to using gamma point calculation with high efficiency

[vega lew]

> Does this mean, that you have single socket quad core CPU/machines, and you 
> have 5 nodes in your cluster? What is the interconnect between the machines?

Yes, my mashines is a single socket on system board. My cpu is intel Core TM II Q6600.
The network between my cluster is only 1Gbit ethernet using TCP/IP.

> Now please have a look on what should be the right value for your setup for 
> G-space/CPU cores and K-points/machines. Hopefully your 'CPU usage' gonna get 
> better.

It's very nice of you saying so. Do your have ever run calculation with the gamma point only via
1Gbit LAN? What about the efficiency? What about the CPU time ? Wall time? How large the system was?

> This depends on your system again (and that quantity what you are interested 
> in). You have to keep in mind, that doing calculations with proper K-point 
> sampling can be several times more expensive than a gamma point only 
> calculations.
> If you get good enough results with Gamma point only with a larger supercell, 
> than it is a happy case. If your system does not have a wide-band gap (like 
> metallic systems and so on)  the Gamma point only results are just far off, 
> and you are forced to do K-point sampling.

My most interested system is the Titanium Oxide surface with 1~3 noble metal atom (too many is more interested,
but DFT can't handle this. I used to calculate from atomic simulation but the force field is not accurate enough)
I think K-point is enough for my ~80 atoms system. But Stefano Baroni tell me 'never trust a truth, unless you  can 
give by yourself a correct answer to a legitimate question'. I think this is quite right. So I'll try 2X2X1 and make a 
energy and geometry comparison with the gamma point only.

> If you do Gamma point only, than you can exploit the parallelization over 
> G-space only. If you wanna use the G-space parallelization over the whole 
> cluster than as i mentioned you need a fast interconnect (infiniband, 
> myrinet, SCI, quadrics and so on). Gigabit LAN is just too slow for that 
> (except for insanely large supercells). 

How could I exploit the parallelization over G-space? Only by use the the proper number of CPU? no pools can be used?
What's the exact meaning of Gigabit LAN is just too slow for that (except for insanely large supercells)? Do you mean I could
calculate with relative high efficiency by using large systems? How many atoms do you call it 'large' supercells?

It's very kind of you to share your experience of PWscf. Thank you so much for the detail discussion.

By the way, are you a German? I like German football team. Let's expect the final of Euro 2008 and the German's excellent
show, even though, Ballack maybe absence. Come on~ 

best wishes.

Vega Lew
PH.D Candidate in Chemical Engineering

State Key Laboratory of Materials-oriented Chemical Engineering
College of Chemistry and Chemical Engineering
Nanjing University of Technology, 210009, Nanjing, Jiangsu, China


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