General molecular mechanics force fields usually employ a large number of molecules as the training set to obtain corresponding force field parameters, which, in turn, can be applied to new molecules. Good transferability enables them to adequately describe various molecular systems with the same set of force field parameters. However, when it comes to specific systems or properties, these general force fields cannot provide a high level of accuracy required by many calculations, such as crystal structure prediction (CSP). By contrast, customized force fields are better positioned to meet the demand for high accuracy with their parameters optimized explicitly for a given molecular system.