A moderate spread of six story buildings might be optimal for energy usage of just the buildings, but total energy usage/environmental impact are also shaped by

A moderate spread of six story buildings might be optimal for energy usage of just the buildings, but total energy usage/environmental impact are also shaped by other human factors for where people want and need to live and work...eg without tall buildings in Manhattan, you would get a lot of extra energy expended in more people commuting back and forth from further distances. And the a high density of people makes public transit even more viable which brings further environmental and health benefits. That said, I do agree that there are only a few areas in the country where the main barrier to an ideal density is allowing more skyscrapers instead of mid-rises. Manhattan north of the financial district and south of midtown is one such area, parts of SF are another. But many, many more cities would see more benefits of moving from low density single-family homes and detached lots to mid-rises.