An atomic bomb works by having a number of neutrons released from the nucleus, and these in turn will dislodge more neutrons from other atoms and so on. Provided the number of neutrons released continues to grow, a nuclear explosion will result. A significant problem is in having enough neutrons released to start the chain reaction going. This will not usually take place with ordinary materials, and the starting material has to be made to release more neutrons than it would do naturally. This is done by compressing the start material, but as you know, trying to compress a solid is difficult. A spherical cage of conventional explosive (shaped charge) is the most common method. Returning to the question, there is no single "equation". If you look up the work of folk like Richard Feynman, and