Citigroup's chief economist, Willem Buiter, believes it to be a certainty that Greece will default on its sovereign debt payments, and he also believes the event will not trigger a financial crisis like the one following the collapses of Lehman Brothers and AIG in 2008.Buiter, who was speaking in Sydney yesterday during a visit to meet the Citigroup's local clients, said: "Everyone knows that Greece will restructure or default. But it is a small nation and its financial situation is simple."Compared with that, Lehman's collapse was unforseen. It was a complex financial institution with subsidiaries around the world. It had complex structures on its balance sheet."When it collapsed no one was sure who owed what to whom. The problem was compounded when AIG collapsed because institutions that thought they had insurance were no longer covered."When a sovereign goes broke the country does not shut down. The country does not go into administration or get broken up and sold off. "What happens is that the sovereign loses access to the capital markets for some time. And that may not be for a long time."The systemic problem is the exposure of the sovereign to strategically important financial institutions. That was seen as a big problem a year ago, but now the same numbers look a lot more acceptable. Europe has the liquidity facilities for this."