One of the main challenges of our time is the energy crisis. We need green energy resources to reduce our dependence on fossil fuels and tackle climate change. Photocatalytic water splitting has been a promising area of research for clean energy applications, as it can potentially provide hydrogen as sustainable alternative fuel from sunlight. To be able to make predictions and design new materials for photocatalysis, we must realistically simulate the water/catalyst interface including the quantum effects and all atomistic interactions. In this talk, I will discuss how we can gain insights via density functional theory (DFT) into the structure of water by studying isotope effects and show how we can capture the competing anharmonic effects by going beyond the harmonic approximation [1-3]. Then I will describe how we can make predictions on potential candidate photocatalysts based on big data analysis for the semiconductors by incorporating approaches beyond standard DFT [4]. Finally, by combining our knowledge of water and catalyst, I will show how we can use ab initio molecular dynamics to gain insights into the water/oxide interfaces [5] and understand the water splitting mechanism to better engineer new interfaces for photocatalytic applications. This talk will demonstrate how recent developments in theoretical calculations allow us to gain deeper understanding into atomistic interactions, for example at the water/oxide interfaces to design new materials for green energy resources.