A method performed by one or more computing systems for identifying a source location of an arrhythmia for a patient based on simulated cardiograms of a simulated human heart, that are similar to a patient cardiogram of a heart of the patient, the patient cardiogram generated based on patient pacing locations of the heart of the patient, the method comprising: generating simulated anatomies of the heart from seed anatomies and sets of weights that include a weight for each seed anatomy, the seed anatomies specify values for dimensions of the heart; for each generated simulated anatomy, running a simulation with a mesh shaped based on the generated simulated anatomy, a simulated pacing location and a source location of an arrhythmia, and generating simulated cardiograms based on simulated electromagnetic output of the simulation; identifying simulated cardiograms whose simulated pacing locations are similar to the patient pacing locations; identifying simulated cardiograms generated based on a simulation configuration parameter that is similar to a patient configuration parameter of the heart of the patient, such as an overall heart size; identifying simulated cardiograms generated based on simulated action potentials that are similar to patient action potentials of the patient; identifying simulated cardiograms generated based on simulated conduction velocities that are similar to patient conduction velocities of the patient; for each of these identified simulated cardiograms, running a patient-specific simulation that is initialized with values of an electromagnetic mesh of the simulated electromagnetic output that generated the respective identified simulated cardiogram and with the patient configuration parameter, and generating a patient-specific simulated cardiogram from electromagnetic output of the simulation; generating a calibrated collection of simulated cardiograms based on the generated patient-specific simulated cardiograms; and identifying as the source location of an arrhythmia for the patient a value of the source location of an arrhythmia associated with the patient-specific simulated cardiograms of the calibrated collection based on similarity of the patient-specific simulated cardiograms to the patient cardiogram. generating simulated anatomies of the heart from seed anatomies and sets of weights that include a weight for each seed anatomy, the seed anatomies specify values for dimensions of the heart; for each generated simulated anatomy, running a simulation with a mesh shaped based on the generated simulated anatomy, a simulated pacing location and a source location of an arrhythmia, and generating simulated cardiograms based on simulated electromagnetic output of the simulation; identifying simulated cardiograms whose simulated pacing locations are similar to the patient pacing locations; identifying simulated cardiograms generated based on a simulation configuration parameter that is similar to a patient configuration parameter of the heart of the patient, such as an overall heart size; identifying simulated cardiograms generated based on simulated action potentials that are similar to patient action potentials of the patient; identifying simulated cardiograms generated based on simulated conduction velocities that are similar to patient conduction velocities of the patient; for each of these identified simulated cardiograms, running a patient-specific simulation that is initialized with values of an electromagnetic mesh of the simulated electromagnetic output that generated the respective identified simulated cardiogram and with the patient configuration parameter, and generating a patient-specific simulated cardiogram from electromagnetic output of the simulation; generating a calibrated collection of simulated cardiograms based on the generated patient-specific simulated cardiograms; and identifying as the source location of an arrhythmia for the patient a value of the source location of an arrhythmia associated with the patient-specific simulated cardiograms of the calibrated collection based on similarity of the patient-specific simulated cardiograms to the patient cardiogram.