HOMO-LUMO ENERGY GAP VERY SMALL!!

[jitmukherjeechem]

My system is a 6,6-single walled carbon nanotube with both ends terminated with hydrogens. When define, I used the default eht parameters. Then i give the molecular charge as zero. A message is shown: WARNING. HOMO-LUMO ENERGY GAP LOWER THAN 0.05!!!!
So,what should I do? Should I reject the default occupation or accept the default and use fermi during optimization....please give me suggestion.

[antti_karttunen]

Hi,

you can accept the default, EHT often yields small gaps with larger or more complicated systems. Just remember to check the occupation after your actual calculation (with eiger-command). Fermi smearing is helpful, but I would first try to converge SCF without it.

Antti

[jitmukherjeechem]

I have to first optimize the molecule and then can do a single point enrgy calculation with fermi. Am I right?

[antti_karttunen]

I would do the Fermi smearing already at the starting geometry to get the occupation right (if there really is need for fermi smearing, that is). Then optimization (with Fermi smearing off) and after convergence just check that the occupation is still OK. But this is just one approach, many others are possible, too.