The CCIS book constitutes selected papers of the 12th International Conference on Computational Science and Computational Intelligence, CSCI 2025, which took place in Las Vegas, NV, USA, during December 3&5, 2025.The 40 full papers included in this book were carefully reviewed and selected from 294 submissions to the Research Track on Large Language Models, CSCI-LLM.They were organized in topical sections as follows: Large Language Models, Knowledge Graphs, Prediction Methods, and Learning Methods; Large Language Models, Natural Language Processing, Security Methods, and Applications; Large Language Models, Data Science, and Related Issues; and Large Language Models, Artificial Intelligence, and Applications.