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Paper text:
dimer and 
 tetramer geometries were optimized in a development version 
 of q - chem at the pbe0-ddsc / def2-svp level , 
 which has been shown to be reliable for noncovalently bound complexes .
the representative grid used to map the contour surface for transfer 
 integrals and binding energies as a function of molecular x- and y - axis translations ( figure 2 ) was constructed from a dimer of bare quaterthiophene 
 optimized at the same level .
the relative monomer barycenters were 
 then displaced ( in x , y ) on intervals 
 of 0.25  at a fix interplane distance ( z ) of 
 3.55  .
the pbe0-ddsc / def2-svp binding energies were computed 
 from the energy difference between the dimer and the monomers in the 
 dimer geometries , with no correction for basis - set superposition error . 

note that the use of other functionals ( e.g. , m06 - 2x ; see supporting information ) leads to 
 the same qualitative features as shown in figure 2b , c .
the charge transfer integrals for hole transport were 
 computed at the pbe0/dzp level for each grid conformation as well 
 as for the pairs of optimized substituted quaterthiophene derivatives . 

we used the direct quantum - mechanical approach implemented in adf 
 2013 that is based on localized monomer orbitals in combination 
 with a basis set orthogonalization procedure .
we 
 show that substituting quaterthiophene cores with strong h - bond 
 aggregators , such as urea groups , provides an efficient way to adjust 
 the mutual in - plane displacements of the semiconducting units and 
 promote charge transfer . 
 our 2-d structure  property mapping 
 reveals that the insertion of substituents induces up to 2.0  
 longitudinal and transversal displacements between the -conjugated 
 moieties . 
 some of these relative displacements lead to improved cofacial 
 orbital overlaps that are otherwise inaccessible due to pauli repulsion . 
 
 our results also emphasize that the fine - tuning of in - plane displacements 
 is more effective than achieving  
 tighter  packing to 
 promote charge - transfer properties .