Hi,

Working with nanotubes I've come across strange problems when trying optimize the interwall distance between the tubes. In my case they are (3,3)'s in a hexagonal arrangement (cross section of the unit cell is a parallelogram, and each corner of the parallelogram contains about a quarter of a nanotube).

1. For some interwall distances, even in 500 datasets the calculation fails to converge while for others it converges nicely within 50.

2. As I decrease the interwall distances over a wide range (2.4 to 3.4 A), the energy just decreases, from -71.12 to below -71.13 while according to the literature it should have a minimum between 3 and 3.2 A.

Can someone offer a suggestion? I'll post my input file below.

Thanks in Advance,

MG

# Number of Data Sets

ndtset 5

# Definition of the unit cell

acell: 6.4 6.4 2.463 angstrom

acell+ 0.1 0.1 0.0 angstrom

rprim 1 0 0

0.5 sqrt(0.75) 0.0

0 0 1

# Definition of the atom types

ntypat 1

znucl 6

ixc 1

# Definition of the atoms

natom 12

typat *1

xangst

1.3082219e+000 -1.5590782e+000 0.0000000e+000

2.0043123e+000 -3.5341431e-001 0.0000000e+000

2.0043123e+000 3.5341435e-001 1.2305009e+000

1.3082219e+000 1.5590782e+000 1.2305009e+000

6.9609033e-001 1.9124925e+000 0.0000000e+000

-6.9609038e-001 1.9124925e+000 0.0000000e+000

-1.3082220e+000 1.5590782e+000 1.2305009e+000

-2.0043123e+000 3.5341431e-001 1.2305009e+000

-2.0043123e+000 -3.5341436e-001 0.0000000e+000

-1.3082219e+000 -1.5590782e+000 0.0000000e+000

-6.9609033e-001 -1.9124925e+000 1.2305009e+000

6.9609037e-001 -1.9124925e+000 1.2305009e+000

# Definition of the planewave basis set

ecut 22

# Definition of the SCF procedure

nstep 500

toldfe 1.0e-12

diemac 12.0

# Usual self-consistent calculation

kptopt 1

shiftk 0 0 0

ngkpt 4 4 4

prtden 1