[QE-users] Behaviour of total force in vc-relax

[Chandler Bennett]

 Hi,

I am seeing something that seems bizarre in a vc-relax calculation
(celldofree='shape') using pw.x. Notice below where I show the total force
and the pressure at each ionic step of the calculation. Notice that at step
14 or so that things seem nearly converged, then suddenly after this point
the magnitude of the pressure begins to drop significantly and the total
force begins to increase significantly. Furthermore, the value of the
pressure that the calculation converges to at the end is *larger* than a
similar calculation that runs with a *smaller* cell volume (everything else
being equal).

Also, for my runs, I have set tprnfor = .true. in my inputs. Are there any
ideas for what is going on here?

     Total force = 0.059366     Total SCF correction = 0.005464    P=
-210.32
     Total force = 0.048261     Total SCF correction = 0.001504    P=
-199.54
     Total force = 0.034713     Total SCF correction = 0.000255    P=
-198.63
     Total force = 0.010340     Total SCF correction = 0.003548    P=
-184.73
     Total force = 0.009330     Total SCF correction = 0.000324    P=
-184.72
     Total force = 0.012479     Total SCF correction = 0.000263    P=
-180.49
     Total force = 0.010120     Total SCF correction = 0.000219    P=
-188.10
     Total force = 0.006841     Total SCF correction = 0.000685    P=
-180.12
     Total force = 0.005409     Total SCF correction = 0.000239    P=
-182.81
     Total force = 0.005971     Total SCF correction = 0.000538    P=
-186.34
     Total force = 0.006673     Total SCF correction = 0.000362    P=
-184.00
     Total force = 0.007914     Total SCF correction = 0.000232    P=
-182.99
     Total force = 0.007067     Total SCF correction = 0.000336    P=
-183.27
     Total force = 0.006471     Total SCF correction = 0.000278    P=
-184.99
     Total force = 0.006406     Total SCF correction = 0.000353    P=
-183.72
     Total force = 0.006868     Total SCF correction = 0.000148    P=
-179.99
     Total force = 0.009121     Total SCF correction = 0.000065    P=
-181.85
     Total force = 0.013930     Total SCF correction = 0.000400    P=
-179.27
     Total force = 0.020806     Total SCF correction = 0.000083    P=
-174.72
     Total force = 0.031915     Total SCF correction = 0.000436    P=
-168.26
     Total force = 0.049062     Total SCF correction = 0.000428    P=
-155.56
     Total force = 0.066521     Total SCF correction = 0.001433    P=
-151.59
     Total force = 0.081470     Total SCF correction = 0.001180    P=
-134.90
     Total force = 0.095677     Total SCF correction = 0.000684    P=
-118.44
     Total force = 0.110083     Total SCF correction = 0.000396    P=
-101.84
     Total force = 0.122165     Total SCF correction = 0.000185    P=
-81.63
     Total force = 0.132184     Total SCF correction = 0.000570    P=
-70.81
     Total force = 0.138722     Total SCF correction = 0.002340    P=
-71.37
     Total force = 0.143747     Total SCF correction = 0.000204    P=
-34.73
     Total force = 0.144336     Total SCF correction = 0.000270    P=
-33.23
     Total force = 0.137834     Total SCF correction = 0.003343    P=
-37.95
     Total force = 0.126686     Total SCF correction = 0.000727    P=
-27.07
     Total force = 0.108605     Total SCF correction = 0.000495    P=
-11.16

-- 

Chandler Bennett
NCSU Department of Physics
Partners III Rm 120D
Cell: (919) 291-6597
mcbennet at ncsu.edu
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