Structure and stability of pristine and modified NaAlH 4 are first investigated using density functional theory (DFT) with plane-wave basis and PW91 functional. Vacancy and Ti-dopant effects in the sodium alanate bulk, (0 0 1) surface, and on-top (0 0 1) surface are then studied considering Na and interstitial lattice sites. Calculated substitution energies of Ti-doped (0 0 1) NaAlH 4 surfaces have shown almost equal probability of substitution at both lattice and interstitial sites. Ti-Al -H complexes are formed depending on the accessible AlH 4 groups around the Ti dopant. TiAl 2 H 7 and TiAl 2 H 2 complexes are found after geometry optimising doped-NaAlH 4 surface models. Their stability and dynamics over time at 423 and 448 K are investigated using periodic density functional molecular dynamics (DFT-MD) simulations. Results have shown increased association of Al and H with the complexes as time evolves. DFT-MD simulations show evolution from TiAl 2 H 7 to TiAl 5 H 7 as time and temperature increase in case of Ti dopant at Na surface site (Ti !