[DFTB-Plus-User] Hessian calculations and time limit

Alessandro LANDI alelandi1 at unisa.it
Mon Feb 24 17:00:47 CET 2020 

Hello,
thank you for your reply. I have attached the input file I am using for
Hessian calculation. I am using the values a senior researcher in my group
suggested me (can you please check the attached file and tell me if you
think they are too strict?)

In particular I set  SCCTolerance = 1e-10. Each geometry step requires
11-15 iSCC cycles, if I understand the output correctly (I reproduce part
of the output in the following)

***  Geometry step: 150
 iSCC Total electronic   Diff electronic      SCC error
    1   -0.13686329E+04    0.00000000E+00    0.21400324E-04
    2   -0.13686329E+04    0.91858965E-10    0.21229086E-04
    3   -0.13686329E+04    0.72759576E-11    0.11813509E-04
    4   -0.13686329E+04   -0.97543307E-10    0.30138133E-05
    5   -0.13686329E+04   -0.21145752E-10    0.74044473E-06
    6   -0.13686329E+04   -0.47748472E-11    0.24149976E-06
    7   -0.13686329E+04   -0.45474735E-12    0.14577856E-06
    8   -0.13686329E+04   -0.68212103E-12    0.38308579E-07
    9   -0.13686329E+04    0.68212103E-12    0.16315258E-07
   10   -0.13686329E+04   -0.45474735E-12    0.57164407E-08
   11   -0.13686329E+04    0.22737368E-12    0.21716895E-08
   12   -0.13686329E+04    0.00000000E+00    0.70770501E-09
   13   -0.13686329E+04   -0.22737368E-12    0.25188207E-09
   14   -0.13686329E+04   -0.22737368E-12    0.10402701E-09
   15   -0.13686329E+04    0.45474735E-12    0.31696423E-10
Total Energy:                    -1328.7955553888 H       -36158.3668 eV
Total Mermin free energy:        -1328.7955553888 H       -36158.3668  eV
Volume:                              0.752806E+05 au^3   0.111554E+05 A^3
Pressure:                           -0.398475E-04 au    -0.117235E+10 Pa

Best regards,
Alessandro

Il giorno lun 24 feb 2020 alle ore 16:51 Mat Toliday via DFTB-Plus-User <
dftb-plus-user at mailman.zfn.uni-bremen.de> ha scritto:

> Hello,
> As far as I know you can not restart the second derivatives calculation.
> However, the Hessian matrix is formed by shifting each atom in the positive
> and negative Cartesian directions and calculating the force. The matrix
> equation is: F_i = H_ij * d_j, where F is force, H is a second derivative,
> d a small displacement  and i and j are indices for the degrees of freedom.
> If you were to add your delta value to an atomic coordinate and then
> calculated the force vector you could then calculate the Hessian entries
> yourself using H_ij = F_i/d_j. You should probably use central finite
> differences, calculating the forces for both + and - d_j values and
> calculate the difference using the forces from the relaxed unit cell. As
> far as I can tell this is what the SecondDerivatives driver does.
> I am a little surprised it is taking too long, what sccTolerance are you
> using and how many SCCcycles does each step take?
> Yours,
> Mat
>
> On Monday, 24 February 2020, 11:17:43 CET, Alessandro LANDI via
> DFTB-Plus-User <dftb-plus-user at mailman.zfn.uni-bremen.de> wrote:
>
>
> Dear DFTB+ users,
> I am trying to calculate the normal modes for a supercell of rubrene
> crystal, consisting of 1120 atoms. I have optimized the supercell and now I
> am computing the Hessian using in the input:
>
> Driver = SecondDerivatives {
>     Atoms = 1:-1
>     Delta = 1E-5
> }
>
> Problem is, I am working on a remote pc where calculations cannot last
> more than three days and it seems that, even using up to 80 cores, it is
> not enough to finish the hessian calculation in time.
> In light of this problem, I am wondering :
> 1) Is it possible to evaluate the second derivatives in more than one run?
> (Like the geometry optimization where I can run a subsequent calculations
> starting from the last geometry of the previous calculation...)
> 2) If this is not possible, can you suggest me another workaround to solve
> this problem?
>
> Thank you for any help you can give me
>
> Best regards,
> Alessandro
>
> --
> Alessandro Landi, Assegnista di Ricerca
> Dipartimento di Chimica e Biologia "Adolfo Zambelli"
> Università degli Studi di Salerno
> Via Giovanni Paolo II, 132 - 84084 - Fisciano (SA)
> Phone number 089969390
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-- 
Alessandro Landi, Assegnista di Ricerca
Dipartimento di Chimica e Biologia "Adolfo Zambelli"
Università degli Studi di Salerno
Via Giovanni Paolo II, 132 - 84084 - Fisciano (SA)
Phone number 089969390
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