1 Original draft for the PCCP journal First principles simulation of reaction steps in the atomic layer deposition of titania: dependence of growth on Lewis acidity of titanocene precursor Aleksandra Zydor 1 , Vadim G. Kessler 2 , Simon D. Elliott 1* 1 Tyndall National Institute, University College Cork, Lee Maltings, Cork, Ireland 2 SLU BioCenter, Department of Chemistry, SLU BioCenter, Box 7015, 75007 Uppsala, Sweden It is a common finding that titanocene-derived precursors do not yield TiO 2 films in ALD with water. For instance, ALD with Ti(OMe) 4 and water gives 0.5 Å/cycle, while TiCp*(OMe) 3 does not show any growth (Me=CH 3 , Cp*=C 5 (CH 3 ) 5 ). From mass spectrometry we found that Ti(OMe) 4 occurs in the gas phase practically exclusively as a monomer. We then used first principles density functional theory (DFT) to model the ALD reaction sequence and find the reason for the difference in growth behaviour. Both precursors adsorb initially via hydrogen-bonding. The simulations reveal that the Cp* * Author to whom correspondence should be addressed: Email: simon.elliott@tyndall.ie, Tel: +353-21- 4904392, Fax: +353-21-4904058