I am trying to model isolated structures such as quantum dots or molecules in Quantum ESPRESSO (QE).
QE uses 3D periodic boundary conditions (PBC) by default. That's why in order to reduce any interaction between the molecule and its images/replicas, we use very large supercells. Now, according to the QE pw.x documentation, we can use the
assume_isolated flag in the
SYSTEM namelist when we want to perform calculation assuming the system to be isolated (a molecule or a cluster in a 3D supercell).
Now, the default setting is
assume_isolated='none' i.e. regular periodic calculation without any correction. But if we set
assume_isolated='makov-payne', then do we also need to set the supercell large enough?
According to this suggestion from Max-centre for Electronic properties of isolated molecules calculation, one should set the
assume_isolated='martyna-tuckerman' and also set the supercell large. My question is why do we need to set the supercell large if we specify the molecule to be isolated? Isolated structure doesn't have any replicas.
Does this mean when
assume_isolated is not set as
'none', then we still have the 3D PBC applied? I am asking this because if setting
assume_isolated='mt' can specify the molecule as isolated, then we don't need to specify the supercell very large and the calculation should be much faster for a small supercell.