# [Pw_forum] VC-relax questions

Timothy Mason thmason24 at yahoo.com
Thu Dec 18 17:00:04 CET 2008

```Gentlemen and ladies,

I have some rookie questions about VC-relax.    I've pasted the input file and greps of Total energy, Total Forces and stresses below.   Its the same problem of oscillations that has been posted before.   Here are my questions.

1).   how can it be that  while the forces and stresses are oscillating the total energy continues to decrease albeit slowly?   Am I really relaxing fine but just have a long way to go?
2)    I've read many posts suggestioning an MD run to determine an effective dt but I don't think I understand.    I run molecular dynamics and look for the oscillation period of the total forces????    Then use a dt equal to some fraction of the period?
3)    I've seen conflicting suggestions on whether the default cell mass is a good guess.      any further thoughts on this?
4)   I'm starting off with "! no symmetries" and hoping to relax to a point where I find symmetries.    is VC-relax even appropriate for this application?

Tim Mason
University of Missouri St. Louis

!    total energy              =  -130.99725337 Ry
!    total energy              =  -130.99742389 Ry
!    total energy              =  -130.99765896 Ry
!    total energy              =  -130.99788028 Ry
!    total energy              =  -130.99814044 Ry
!    total energy              =  -130.99838484 Ry
!    total energy              =  -130.99862662 Ry
!    total energy              =  -130.99883535 Ry
!    total energy              =  -130.99906028 Ry
!    total energy              =  -130.99921630 Ry
!    total energy              =  -130.99942817 Ry
!    total energy              =  -130.99962083 Ry
!    total energy              =  -130.99984032 Ry
!    total energy              =  -131.00005894 Ry
!    total energy              =  -131.00027167 Ry
!    total energy              =  -131.00032697 Ry
!    total energy              =  -131.00055195 Ry
!    total energy              =  -131.00075005 Ry
!    total energy              =  -131.00092926 Ry
!    total energy              =  -131.00110579 Ry
!    total energy              =  -131.00128893 Ry
!    total energy              =  -131.00152437 Ry
!    total energy              =  -131.00176666 Ry
!    total energy              =  -131.00201744 Ry
!    total energy              =  -131.00222657 Ry

Total force =     0.025775     Total SCF correction =     0.000281
Total force =     0.020098     Total SCF correction =     0.000361
Total force =     0.015661     Total SCF correction =     0.000081
Total force =     0.016340     Total SCF correction =     0.000253
Total force =     0.012358     Total SCF correction =     0.000853
Total force =     0.012918     Total SCF correction =     0.000221
Total force =     0.011349     Total SCF correction =     0.000287
Total force =     0.013804     Total SCF correction =     0.000249
Total force =     0.012754     Total SCF correction =     0.000124
Total force =     0.016969     Total SCF correction =     0.000230
Total force =     0.011474     Total SCF correction =     0.000262
Total force =     0.013177     Total SCF correction =     0.000210
Total force =     0.010928     Total SCF correction =     0.000265
Total force =     0.009784     Total SCF correction =     0.000097
Total force =     0.010154     Total SCF correction =     0.000534
Total force =     0.023392     Total SCF correction =     0.000539
Total force =     0.018644     Total SCF correction =     0.000303
Total force =     0.013266     Total SCF correction =     0.000284
Total force =     0.012565     Total SCF correction =     0.000137
Total force =     0.011408     Total SCF correction =     0.000406
Total force =     0.013103     Total SCF correction =     0.000212
Total force =     0.011107     Total SCF correction =     0.000195
Total force =     0.009844     Total SCF correction =     0.000492
Total force =     0.010155     Total SCF correction =     0.000692
Total force =     0.014920     Total SCF correction =     0.000309

total   stress  (Ry/bohr**3)                   (kbar)     P=    0.85
total   stress  (Ry/bohr**3)                   (kbar)     P=    1.09
total   stress  (Ry/bohr**3)                   (kbar)     P=    1.44
total   stress  (Ry/bohr**3)                   (kbar)     P=    2.07
total   stress  (Ry/bohr**3)                   (kbar)     P=    2.36
total   stress  (Ry/bohr**3)                   (kbar)     P=    2.74
total   stress  (Ry/bohr**3)                   (kbar)     P=    2.96
total   stress  (Ry/bohr**3)                   (kbar)     P=    2.95
total   stress  (Ry/bohr**3)                   (kbar)     P=    2.92
total   stress  (Ry/bohr**3)                   (kbar)     P=    3.35
total   stress  (Ry/bohr**3)                   (kbar)     P=    3.21
total   stress  (Ry/bohr**3)                   (kbar)     P=    3.13
total   stress  (Ry/bohr**3)                   (kbar)     P=    2.91
total   stress  (Ry/bohr**3)                   (kbar)     P=    2.85
total   stress  (Ry/bohr**3)                   (kbar)     P=    2.61
total   stress  (Ry/bohr**3)                   (kbar)     P=    1.46
total   stress  (Ry/bohr**3)                   (kbar)     P=    1.75
total   stress  (Ry/bohr**3)                   (kbar)     P=    2.29
total   stress  (Ry/bohr**3)                   (kbar)     P=    2.03
total   stress  (Ry/bohr**3)                   (kbar)     P=    1.97
total   stress  (Ry/bohr**3)                   (kbar)     P=    2.01
total   stress  (Ry/bohr**3)                   (kbar)     P=    2.26
total   stress  (Ry/bohr**3)                   (kbar)     P=    2.44
total   stress  (Ry/bohr**3)                   (kbar)     P=    2.66
total   stress  (Ry/bohr**3)                   (kbar)     P=    2.59

&CONTROL
calculation = "vc-relax",
prefix = 'n1',
tprnfor = .TRUE.
pseudo_dir = '/home/tim/PWSCF/PSPS',
outdir='/home/tim/tmp'
etot_conv_thr = 1.0D-5,
forc_conv_thr = 0.0004,
dt = 20
nstep = 100
/

&SYSTEM
ibrav = 0, nat = 15, ntyp = 5,
ecutwfc = 40
ecutrho = 480
celldm(1) = 1.889725989
/

&ELECTRONS
mixing_mode = 'plain',
mixing_beta = 0.7,
conv_thr = 1.0D-7,
/

&IONS
ion_dynamics  = 'damp',
ion_damping  = 0.1,
ion_positions = 'from_input',
pot_extrapolation = "second_order",
wfc_extrapolation = "second_order",
upscale = 100,
/

&CELL
press = 0.0,
cell_dynamics = 'damp-w'
/

ATOMIC_SPECIES
Li 1.0 Li.pbe-n-van.UPF
Ca 1.0 Ca.pbe-nsp-van.UPF
N 1.0 N.pbe-rrkjus.UPF
H 1.0 H.pbe-rrkjus.UPF
B 1.0 B.pbe-n-van.UPF

CELL_PARAMETERS
11.03225111 0.49197118 -0.92807585
-0.15038487 5.49373010 -0.38883048
3.04894498 0.25407718 4.19633759

ATOMIC_POSITIONS {crystal}
Li 0.144228066000 0.155479905000 0.105275909000
Ca 0.911201719000 0.659605393000 0.179405853000
N 0.954182418000 0.066667349000 0.203452528000
H 0.879987796000 0.160521652000 0.398773311000
H 0.949093887000 0.150685763000 0.033727675000
H 0.590246767000 0.244749224000 0.483311768000
H 0.563997731000 0.067035740000 0.225636766000
H 0.437455407000 0.306433941000 0.454794956000
H 0.611216941000 0.379448807000 0.154202770000
H 0.231109967000 0.728337975000 0.002905585000
H 0.397236924000 0.644804127000 -0.037067236000
H 0.288752224000 0.877300760000 0.238936122000
H 0.231627436000 0.575300194000 0.301639728000
B 0.550495441000 0.249323405000 0.329678852000
B 0.288025902000 0.707564656000 0.123296526000

K_POINTS automatic
4 4 4 0 0 0

-------------- next part --------------
An HTML attachment was scrubbed...