Calpine\u2019s board rebuffed NRG Energy\u2019s takeover attempt May 30. NRG\u2019s attempt involves using its capital to merge with an energy firm that would lighten its carbon heavy footprint. According to analysts, however, Calpine didn\u2019t slam the door on NRG\u2019s foot. Calpine stated the two companies are assessing whether there\u2019s a \u201cbasis for discussion . . . to explore a business combination.\u201d NRG is \u201crepositioning in a carbon-constrained world,\u201d according to company documents. NRG owns aging natural gas plants in California, coal-fired plants, and is exploring nuclear power. Calpine has considerable geothermal generating capacity in California, making it a sizeable renewable energy player. Its main generation is also from natural gas-fired plants, which are cleaner than coal. It emerged from bankruptcy reorganization in January. In its most recent Securities and Exchange Commission filing, it notes the company has power contracts, at a loss, in the $855 million range. It has about $281 million in cash. Last year it had about $1.5 billion in cash. NRG estimates Calpine\u2019s operating losses to be $5.1 billion, in its SEC filing. Another potential for takeover in California could be Mirant, which is also on shaky financial ground, according to its last quarterly report. The Atlanta-based energy company\u2019s profits thinned to $16 million in the fourth quarter of 2007, compared to $1.3 billion net income in fourth quarter 2006. However, Mirant is not a prime target because it owns older power plants with a larger carbon footprint, according to a source.