  Two-dimensional transition metal dichalcogenides are among the most promising
materials for water-splitting catalysts. While a variety of methods have been
applied to promote the hydrogen evolution reaction on the transition metal
dichalcogenides, doping of transition metal heteroatoms have attracted much
attention since it provides effective ways to optimize the hydrogen adsorption
and H2 generation reactions. Herein, we provide in-depth and systematic
analyses on the trends of the free energy of hydrogen adsorption ({\Delta}GH*),
the most well-known descriptor for evaluating hydrogen evolution reaction
performance, in the doped transition metal dichalcogenides. Using the total 150
doped transition metal dichalcogenides, we carried out the atom-level analysis
on the origin of {\Delta}GH* changes upon the transition metal heteroatom
doping, and suggest two key factors that govern the hydrogen adsorption process
on the doped transition metal dichalcogenides: 1) the changes in the charge of
chalcogen atoms where hydrogen atoms adsorbed for the early transition metal
doped structures, and 2) the structural deformation energies accompanying in
introduced dopants for the late transition metal doped structures. Based on our
findings, we interpret from a new perspective how vacancies in the TM-doped
TMDs can provide optimal {\Delta}GH* in HER. We suggest electrostatic control
for early TM doped systems and structural control for late TM doped systems as
the effective strategies for the thermoneutral {\Delta}GH* in TMD.
