The Pentagon is paying a California research company to "determine the nuclear weapon employment strategy that would eliminate U.S.S.R. as a functioning national entity," according to Defense Department documents.

In Vienna, Va., another Pentagonfinanced consulting firm is looking at "the viability of employing strategic nuclear weapons to achieve regionalization of the Soviet Union" -- which one Defense Department official described last week as an attack that would destroy regional areas that support the present Soviet government and "unleash forces of separatism."

The same Virginia firm is also exploring for the Pentagon "strategic targeting against Soviet leadership," described by the Defense official as an effort to "paralyze, disrupt or dismember" the Russian government by wiping out its entire ruling group.

These three long-term Pentagonfinanced research projects are among dozens of studies started last yeat when the White House and Defense Department decided to look at alternative ways for targeting the massive U.S. startegic force that now contains 8,000 warheads and is expected to grow to 12,000 warheads by the late 1980s.

The decision to promote thest particulat types of blue-sky research represents a significant change in the way the U.S. government looks at nuclear weapons and nuclear war.

Up to now, analysts have focused nuclear targeting on destruction of Soviet missiles and other military installations along with Russia's population centers and industrial base.

Past administrations have looked upon a major nuclear exchange between the two superpowers as so destructive that neither side could survive. The threat of such an event was the best deterrent to its happening.

The new studies, however, may pave the way for a change in that philosophy, if not by President Carter, at least by the Defense Department.

"We have never really thought the thing through," one of the Pentagon researchers said last week.

Said Defense Department official involved in the projects, "We are trying to see in the ultimate nuclear exchange, what should we be trying to do other than just flatten their industry?"

"It's thinking the unthinkable," one study participant said. "We don't want to bomb them into the Stone Age.We should have real objectives that make sense."

In short, the officials who promoted the studies and the firms that are doing them already think a nuclear war is possible. If one occurs, they believe the United States should have a realistic targeting strategy for winning it.

The $78,499 fiscal 1979 contract with Analytical Assessment Corp., (Anasmc/) of Marina Del Rey, Calif., offers an opportunity to explore the thinking behind the new research. ANASMC officials Abe Wagner and Howard Berger said recently they are in their second year of trying to develop a strategy to end Russia as "a national entity" and are still asking, "Is it possible?"

Using physicists, economists, mathematicians and a host of other experts, they are studying what the Soviet Union would be like after an attack by U.S. missiles as they are now targeted.

They say the number of deaths "would not be 100 million... There would not be total destruction... Their economy does survive although transportation and communications might not."

Their next step will be to look at whether the surviving elements would be enough to keep the Soviet state alive. If they would, the researchers expect to go back anddetermine what other targets could have been hit and if they could end the Soviet state's existence.

"If we plan to win" in a nuclear exchange, one of the ANASMC researchers said last week, "we would want them to be able to recover quickly" since responsiblity for getting the surviving Russian people back on their feet would be a U.S. burden.

Thus, he went on, "we see a nuclear strategy that calls for something less than wiping them out."

His and the other studies in this target research are aimed primarily at "bringing about the collapse of the [Soviet] government that now exists" but without massive destruction to that country.

Other studies in this series include:

A comprehensive look at Soviet techological foundations and their potential as nuclear targets.

Determination of vulnerabilities of Soviet electric power stations to nuclear attack.

Study of nuclear weapons Effects on the Soviet chemical industry.

Results of these long-range studies will be circulated among key Defense Department officals and reviewed as "possible alternatives to current strategies," a Pentagon offical said last week.

But before any are adopted, they would have to go through extensive further study, he said.