Atheist: Objective morality can be arrived at with reason alone and we should all be obligated to live by it. Right is simply what makes the most people happy.
Response: I am not sure you understand the implications of your assertions. Given your worldview you have no reason to believe in a moral imperative that is true for everyone. Who decides? And to whom is everyone accountable? And by what standard? In an accidental universe, whatever happens, just happens. Chemical reactions simply follow the law of physics.
To claim that morality is OBJECTIVE is like saying that they are like the laws of logic, which are true for everyone, regardless of where they were born and what culture they were brought up in. The law of non-contradiction, for example, is true regardless of whether you were born in Mexico or Indonesia. And, as you know, these abstract and immaterial laws of logic do not change with time and are always true without exception. Is that what you mean by objective morality? And what is to prevent your neighbor from creating a different morality than you? Why should your view be the accepted view?
In an accidental universe why would there be universal standards? Therefore it is quite plain that morality could never be objective in a strictly atheistic material universe. To claim something like right is something that makes the most people happy is to make an arbitrary assertion. Why should people be obligated to follow THAT particular preference? And even if they did, they might have a different definition of happiness. And how could one chemical reaction be morally obligated to another? If we are merely the reaction of chemistry over time why does anyone deserve respect?