Purpose of d3hess.x: This is a post processing program to compute second derivatives of D3 dispersion in QE. The d3hess.x program should be run after an scf or relax calculation. It reads the molecular geometry from the xml file in the outdir and save the second derivatives matrix in a file on the disk. Afterwords, phonon reads the file and add the D3 Hessian matrix to the dynamical matrix with the proper phase (q), to include dispersion effects on vibrational frequencies. The workflow is just: (1) do an SCF (2) run d3hess (3) run phonon Please note that filhess in d3hess input and dftd3_hess in phonon input, if given, should match. Please also note that second derivatives of the three-body term of d3 dispersion are not implemented, and phonon calculations with d3 should be run with dftd3_threebody=.false. in the SCF. Structure of the input data: ============================ &INPUT ... / 'pwscf' prefix of input file produced by pw.x (wavefunctions are not needed) directory containing the input data, i.e. the same as in pw.x value of the ESPRESSO_TMPDIR environment variable if set; current directory ('./') otherwise 'prefix.hess' output file where the D3 hessian matrix is written (should match dftd3_hess keyword in phonon calculation) step for numerical differentiation in a.u. 1.d-3