Geneva talks, part of a wider thaw?

What do you say after you've exchanged the opening pleasantries? It won't be an easy question for either Soviet Foreign Minister Andrei Gromyko or US Secretary of State George Shultz when they meet in Geneva on Jan. 7 and 8.

Some analysts here speculate that a mutual ''sounding out'' is about all that is likely to be accomplished in the two days of talks. Still, as one diplomat says, ''to get Shultz and Gromyko to sit down and talk for two days is very, very important.''

In fact, there is a nascent hope here in Moscow that the end of icy relations between the Soviet Union and the United States is finally in sight. And analysts give credit for this to a mixture of motives - ranging from President Reagan's reelection, which leaves Moscow with the stark choice of negotiating or facing four more years of an American arms buildup, to Soviet leader Konstantin Chernenko's current burst of activity.

Chernenko, says one diplomat, has ''come on like gangbusters this fall.''

Kremlin-watchers here, however, stress that a real breakthrough won't come quickly or easily.

A Western diplomat here admits that many problems still divide the two countries.

Yet, he adds, ''I do not think these two governments would sign an agreement to enter into new negotiations if both sides did not have some hope of success.''

Georgi Arbatov, the Kremlin's chief strategist on policy toward the US, gave voice to similar views.

''I think it's going to take time,'' he told reporters outside his office.

There could be a return to the earlier spirit of ''detente'' between the superpowers, he said, ''if there is a political will in Washing-ton.''

Is the agreement, then, a ''breakthrough''?

''Breakthrough,''

says one Western European diplomat, ''is not a term I would have used.''

A simple, two paragraph statement released simultaneously in Washington and Moscow on Thanksgiving Day said that the two countries had agreed ''to enter new talks with the aim of achieving mutually acceptable accords on the entire complex of questions concerning nuclear and space weapons.''

After reading those words to reporters jammed into a central Moscow conference room, Kremlin spokesman Vladimir Lomeiko laid particular stress on certain passages. ''This is not a renewal of negotiations,'' he said. ''It is about new talks, absolutely new negotiations.''

The Soviets walked out on two sets of negotiations in Geneva last year, on medium-range and long-range nuclear missiles. The event that sparked their departure was the deployment of new American medium-range missiles in Europe, under an agreement forged within the NATO alliance.

For a year, the Soviets refused to talk with the US on nuclear armaments, insisting that the new NATO missiles first be removed before negotiations could be restarted. The US did not comply.

Western analysts hope that the Soviet stress on ''new'' talks is a face-saving formulation to get around their vow not to return to the previous set of talks. If so, it could represent a new Soviet willingness to compromise - made easier by US willingness to talk about the issue of space-based weapons.

The ''militarization of outer space'' is a key concern of the Soviets. But up to now the US has been loath to join in discussions on this because of a Soviet insistence that there first be a moratorium on US space weapons testing. US officials have insisted that a moratorium would impede testing for Mr. Reagan's proposed ''strategic defense initiative'' (dubbed the ''star wars'' program).

Notably, the Soviet demand for a moratorium now seems to have been dropped - at least from the public statements. And Western diplomats here say there are no private agreements that amount to a de facto moratorium, either.

So, it appears, the Soviets have given up a lot to go the negotiating table in Geneva. But Kremlin spokesman Lomeiko made it clear that some of the lingering problems facing the superpowers - notably, the new American intermediate-range missiles in Europe - would be brought up anew at the Shultz-Gromyko talks in Geneva.

Some diplomats here theorize that the Soviets will initially be trying to determine whether the American side has changed its stance on any of the issues dividing the two countries - whether the Reagan administration will still resist a moratorium on space weapons testing, and whether it will still refuse to consider a pullback of the new NATO missiles in Europe.

And the Americans will also be testing Soviet resolve on the very same issues. Hence, the mutual ''sounding out.''

One key question is whether the two diplomats will keep the questions of long- and short-range nuclear missiles and space-based weapons as part of one over-reaching set of talks, or split them up in hopes of tackling each issue independently.

One analyst here says that the Soviets have clearly linked negotiations on intermediate- and long-range missiles - that is, missiles to be used only within Europe and bigger ones that fly between continents.

''I have seen nothing to suggest that basic approach has changed,'' he says.

What, then, has changed?

One obvious explanation for the Kremlin's willingness to sit down in Geneva may simply be the outcome of the US election with its strong mandate for President Reagan.

But analysts here say the internal machinations of the Kremlin leadership have inevitably played a role, too - including Mr. Chernenko's return to vigorous activity since his return from an extended absence in late summer.

Indeed, several analysts say Chernenko appears to have become much more assertive - even ''feisty'' - in the past few months. Whether this is the result of Kremlin image-building, or growing actual authority, remains unclear.

There is also another notable development, that may or may not be related to the Kremlin's willingness to negotiate with the US.

Communist Party officials have let it be known that this week, for the first time in years, an important Kremlin tradition will be changed. The Communist Party's Central Committee is usually convened here in Moscow for a plenum in advance of the year-end session of the country's nominal parliament, the Supreme Soviet.

The Supreme Soviet is meeting this week as scheduled, but no Central Committee meeting has been called.

That could be a sign of Mr. Chernenko's increasing willingness to assert his own leadership and revert to an earlier practice of only convening the Central Committee to discuss specific issues - and not before every parliamentary session.

It also might be that Chernenko has too many potentially divisive issues on his hands right now to want to face questions from the Central Committee. These include the possible need to find a replacement for Soviet Defense Minister Dmitri Ustinov, who is reportedly ailing.

And, of course, Chernenko may simply not have wanted to offer a justification for the decision to have Mr. Shultz meet with Mr. Gromyko - an item that inevitably would have dominated much of the discussion. Still, there is little speculation that Soviet Foreign Minister Andrei Gromyko's power base has eroded.

''Gromyko continues to be in a very strong position. . . . He continues to be very powerful,'' says one analyst.

Another adds that he continues to be ''a very conservative fellow.''

Still, Gromyko also undoubtedly knows the art of strategic compromise.

Within the Kremlin, says one diplomat, ''There's often a trade-off between things that are important and things that are possible.''

Given the current state of relations between the US and the Soviet Union, the case for that which is possible - rather than that which the Kremlin wanted - may have seemed stronger.

When the two sides sit down in Geneva in January, says one analyst, the items on the Soviet agenda will give further indication of how the Kremlin would like to steer the course of superpower relations over the next four years of a Reagan presidency.