This is way off the modeling track, but I'll give you guys the latest update.
Cessna 172 was heading north and climbing out of Centennial Airport which is at the southern part of the city. The time is shortly after sunset. He's slightly lower than the twin and climbing out of 7300 ft receiving permission from ATC to go 8000 ft.
Piper Apache heading south maintaining roughly level altitude and slightly above the Cessna (7600 to 7800 feet). For some reason the Piper was not transponding altitude but the pilot was asked for altitude by Denver air traffic and provided information as requested.
Both aircraft were being tracked by Denver International Airport traffic control. Both aircraft were warned of the other at approx. 1 mile separation (not very much distance). The Piper made an evasive maneuver at the last minute. The Cessna aparently didn't make any adjustments. The two aircraft clipped wings head on.
The Piper went inverted and did a flat spin to the ground. The Cessna lost half it's wing, nosed straight in. Both crashed in a crowded neighborhood where houses are very close together. Amazingly only a few people on the ground were injured, and that was because they were too close to the aircraft after the fuel exploded.
The evidence indicates that lighting, pilot error, malfunctioning or not operating transponder, and not enough warning given by ATC all played a part in this terrible accident. The lawyers should stay busy on this one for years.
Take care and safe flying