CREST interface

ASH features a simple interface to the powerful conformational sampling program crest by the Grimme group.

By providing an ASH fragment object to the call_crest function, the Crest xTB-metadynamics-based conformational sampling procedure is invoked. The output from crest is written to standard output. If successful, Crest will create a file that can be directly read into ASH for further processing or further calculations. This allows one to write a multi-step workflow of which the crest-procedure is one of many steps.

#Function to call crest
def call_crest(fragment=None, xtbmethod=None, crestdir=None,charge=None, mult=None, solvent=None, energywindow=6, numcores=1,
               constrained_atoms=None, forceconstant_constraint=0.5)
#Function to grab conformers. Returns list of conformers and list of xtb energies
def get_crest_conformers()

call_crest requires one to specify: an ASH fragment, xtbmethod (GFN1-xTB or GFN2-xTB ), location of crest directory, charge, multiplicity.

Optional keywords are: solvent, energywindow (default 6), numcores (default 1), constrained_atoms (list of integers) and the value of the force-consstant.

If you specify a list of constrained atoms then ASH will create an .xcontrol file that defines the constraints according to crest Example Applications.

Example workflow 1. Call crest to get low-energy conformers as ASH fragments.

from ash import *


#0. Starting structure and charge and mult
molecule = Fragment(xyzfile="", charge=0, mult=1)

#1. Calling crest and getting list of conformer fragments and energies
list_conformer_frags, xtb_energies = call_crest(fragment=molecule, xtbmethod='GFN2-xTB', crestdir=crestdir,

print("list_conformer_frags:", list_conformer_frags)
print("Crest Conformer Searches done. Found {} conformers".format(len(xtb_energies)))
print("xTB energies: ", xtb_energies)

confsampler_protocol : Automatic Crest+DFTopt+DLPNO-CCSD(T) workflow

It is also possible to call the confsampler_protocol function that carries out an automatic multi-step workflow at various levels of theory.

  1. conformational sampling using crest and GFN-xTB (low-level theory).

  2. Geometry optimizations for each low-energy conformer at a medium-level of theory (typically DFT using e.g. ORCATheory)

  3. High-level single-point calculation (e.g. DLPNO-CCSD(T)/CBS using e.g. ORCA_CC_CBS_Theory)

def confsampler_protocol(fragment=None, crestdir=None, xtbmethod='GFN2-xTB', MLtheory=None,
                        HLtheory=None, numcores=1, charge=None, mult=None, crestoptions=None,
from ash import *

#Fragment to define
frag=Fragment(xyzfile="", charge=0, mult=1)

#Defining MLTheory: DFT optimization
MLsimpleinput="! B3LYP D3BJ def2-TZVP TightSCF "
%scf maxiter 200 end
ML_B3LYP = ORCATheory(orcasimpleinput=MLsimpleinput, orcablocks=MLblockinput, numcores=numcores)
#Defining HLTheory: DLPNO-CCSD(T)/CBS
HL_CC = ORCA_CC_CBS_Theory(elements=frag.elems, cardinals = [2,3], basisfamily="def2", DLPNO=True,
    pnosetting='extrapolation', pnoextrapolation=[6,7], numcores=numcores)

#Call confsampler_protocol
confsampler_protocol(fragment=frag, crestdir=crestdir, xtbmethod='GFN2-xTB', MLtheory=ML_B3LYP,
                         HLtheory=HL_CC, orcadir=orcadir, numcores=numcores)

Final result table of calculated conformers at 3 different theory levels:


 Conformer   xTB-energy    DFT-energy    HL-energy (Eh)
         0 -25.8392205500 -346.2939482921 -345.2965932205
         1 -25.8377914500 -346.2884905132 -345.2911748671
         2 -25.8358803400 -346.2818766960 -345.2848279253
         3 -25.8313250600 -346.2788608396 -345.2815202116
         4 -25.8307377800 -346.2788662649 -345.2815419285
         5 -25.8303374700 -346.2775476223 -345.2792917601
         6 -25.8300128900 -346.2776089771 -345.2794648759

 Conformer   xTB-energy    DFT-energy    HL-energy (kcal/mol)
         0  0.0000000000  0.0000000000  0.0000000000
         1  0.8967737821  3.4248079602  3.4000680178
         2  2.0960134034  7.5750408530  7.3828340833
         3  4.9544947374  9.4675192805  9.4584557521
         4  5.3230184983  9.4641148891  9.4448282319
         5  5.5742168139 10.2915756050 10.8568301896
         6  5.7778938373 10.2530749008 10.7481984235