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

[Mat Toliday]

 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)
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