[Pw_forum] negative dr2

[Dan Fors]

Dear users,

I am running a simple 2-atomic non-magnetic (NM) bcc Cr system (see
below for the input file)  with v.4.2.1 and I keep getting an error
message regarding a negative dr2:

     from mix_rho : error #         1
     negative dr2

This error occurs for values close to as well as away from the
estimated equilibrium volume. What is the possible the origins of this
error and how may I eliminate it?

I have tried to change the mixing_beta in the range 0.1 to 0.8, as
well as changing the mixing mode to 'TF'. I have also tried to set
nspin = 2 with a zero start magnetisation since an anti-ferromagnetic
(AFM) start configuration does not give rise to the problem (but
yields the AFM solution instead of the NM one). However. this also
gives the error. Finally, I tested to use ibrav = 3 and with only one
atom in the cell, but I still get the above error.

What I have noticed is that in the AFM case I get a couple of very
flat bands in the bandstructure just above the Fermi level. If I
change to a USPP potential or an alternative PAW potential these bands
disappear, and the above error does not occur. Can the potential I use
in the present case be the source to the error, and if yes, what is
the reason behind it?

I would appreciate any help or suggestions.

Dan



%-------------------------------------

&CONTROL
	calculation = 'scf',
	restart_mode = 'from_scratch',
	prefix = 'Cr',
	tstress = .true.,
	tprnfor = .true.,
	pseudo_dir = '/home/forsdan/PROGRAMS/QUANTUM_ESPRESSO/espresso-4.2.1_intel/pseudo/',
	outdir = '/home/forsdan/FeCr_PROJECT/TEMP_PAW/'
/
&SYSTEM
	ibrav =  1, celldm(1) = 5.378798, nat =  2, ntyp = 1,
	ecutwfc = 40.0, ecutrho = 180.0, nbnd = 14
	occupations = 'smearing',
	smearing = 'mp', degauss = 0.022,
	nspin = 1,
/
&ELECTRONS
	electron_maxstep = 100,
	conv_thr =  1.0d-8,
	diagonalization = 'cg',
	mixing_mode = 'plain',
	mixing_beta = 0.3
/
ATOMIC_SPECIES
 Cr  51.996  Cr.pbe-paw_kj_6.UPF
ATOMIC_POSITIONS {crystal}
 Cr 0.00 0.00 0.00
 Cr 0.50 0.50 0.50
K_POINTS {automatic}
 12 12 12 0 0 0

%-------------------------------------